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Theorem List for Metamath Proof Explorer - 27301-27400   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremee3bir 27301 Right-biconditional form of e3 27546 without virtual deduction connectives. Special theorem needed for Alan Sare's virtual deduction translation tool. (Contributed by Alan Sare, 22-Jul-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   &    |-  ( ta  <->  th )   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  ta ) ) )
 
Theoremee13 27302 e13 27557 without virtual deduction connectives. Special theorem needed for Alan Sare's virtual deduction translation tool. (Contributed by Alan Sare, 28-Oct-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ps )   &    |-  ( ph  ->  ( ch  ->  ( th  ->  ta ) ) )   &    |-  ( ps  ->  ( ta  ->  et ) )   =>    |-  ( ph  ->  ( ch  ->  ( th  ->  et ) ) )
 
Theoremee121 27303 e121 27462 without virtual deductions. (Contributed by Alan Sare, 13-Jul-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ps )   &    |-  ( ph  ->  ( ch  ->  th )
 )   &    |-  ( ph  ->  ta )   &    |-  ( ps  ->  ( th  ->  ( ta  ->  et )
 ) )   =>    |-  ( ph  ->  ( ch  ->  et ) )
 
Theoremee122 27304 e122 27459 without virtual deductions. (Contributed by Alan Sare, 13-Jul-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ps )   &    |-  ( ph  ->  ( ch  ->  th )
 )   &    |-  ( ph  ->  ( ch  ->  ta ) )   &    |-  ( ps  ->  ( th  ->  ( ta  ->  et )
 ) )   =>    |-  ( ph  ->  ( ch  ->  et ) )
 
Theoremee333 27305 e333 27542 without virtual deductions. (Contributed by Alan Sare, 17-Jul-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   &    |-  ( ph  ->  ( ps  ->  ( ch  ->  ta ) ) )   &    |-  ( ph  ->  ( ps  ->  ( ch  ->  et )
 ) )   &    |-  ( th  ->  ( ta  ->  ( et  ->  ze ) ) )   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  ze )
 ) )
 
Theoremee323 27306 e323 27575 without virtual deductions. (Contributed by Alan Sare, 17-Apr-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   &    |-  ( ph  ->  ( ps  ->  ta )
 )   &    |-  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) )   &    |-  ( th  ->  ( ta  ->  ( et  ->  ze )
 ) )   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  ze ) ) )
 
Theorem3ornot23 27307 If the second and third disjuncts of a true triple disjunction are false, then the first disjunct is true. Automatically derived from 3ornot23VD 27657. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( -.  ph  /\  -.  ps )  ->  ( ( ch  \/  ph  \/  ps )  ->  ch ) )
 
Theoremorbi1r 27308 orbi1 689 with order of disjuncts reversed. Derived from orbi1rVD 27658. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  <->  ps )  ->  (
 ( ch  \/  ph ) 
 <->  ( ch  \/  ps ) ) )
 
Theorembitr3 27309 Closed nested implication form of bitr3i 244. Derived automatically from bitr3VD 27659. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  <->  ps )  ->  (
 ( ph  <->  ch )  ->  ( ps 
 <->  ch ) ) )
 
Theorem3orbi123 27310 pm4.39 846 with a 3-conjunct antecedent. This proof is 3orbi123VD 27660 automatically translated and minimized. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ( ph  <->  ps )  /\  ( ch 
 <-> 
 th )  /\  ( ta 
 <->  et ) )  ->  ( ( ph  \/  ch 
 \/  ta )  <->  ( ps  \/  th 
 \/  et ) ) )
 
Theoremsyl5imp 27311 Closed form of syl5 30. Derived automatically from syl5impVD 27673. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  ->  ( ps 
 ->  ch ) )  ->  ( ( th  ->  ps )  ->  ( ph  ->  ( th  ->  ch )
 ) ) )
 
Theoremimpexp3a 27312 The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. After the User's Proof was completed it was minimized. The completed User's Proof before minimization is not shown. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
1::  |-  ( ( ( ps  /\  ch )  ->  th )  <->  ( ps  ->  ( ch  ->  th ) ) )
qed:1:  |-  ( ( ph  ->  ( ( ps  /\  ch )  ->  th ) )  <->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
 |-  (
 ( ph  ->  ( ( ps  /\  ch )  ->  th ) )  <->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
 
Theoremcom3rgbi 27313 The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
1::  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  th ) ) )  ->  ( ph  ->  ( ch  ->  ( ps  ->  th ) ) ) )
2::  |-  ( ( ph  ->  ( ch  ->  ( ps  ->  th ) ) )  ->  ( ch  ->  ( ph  ->  ( ps  ->  th ) ) ) )
3:1,2:  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  th ) ) )  ->  ( ch  ->  ( ph  ->  ( ps  ->  th ) ) ) )
4::  |-  ( ( ch  ->  ( ph  ->  ( ps  ->  th ) ) )  ->  ( ph  ->  ( ch  ->  ( ps  ->  th ) ) ) )
5::  |-  ( ( ph  ->  ( ch  ->  ( ps  ->  th ) ) )  ->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
6:4,5:  |-  ( ( ch  ->  ( ph  ->  ( ps  ->  th ) ) )  ->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
qed:3,6:  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  th ) ) )  <->  ( ch  ->  ( ph  ->  ( ps  ->  th ) ) ) )
 |-  (
 ( ph  ->  ( ps 
 ->  ( ch  ->  th )
 ) )  <->  ( ch  ->  (
 ph  ->  ( ps  ->  th ) ) ) )
 
Theoremimpexp3acom3r 27314 The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
1::  |-  ( ( ph  ->  ( ( ps  /\  ch )  ->  th ) )  <->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
2::  |-  ( ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  <->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
qed:1,2:  |-  ( ( ph  ->  ( ( ps  /\  ch )  ->  th ) )  <->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
 |-  (
 ( ph  ->  ( ( ps  /\  ch )  ->  th ) )  <->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
 
Theoremee1111 27315 Non-virtual deduction form of e1111 27481. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.) The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown.
h1::  |-  ( ph  ->  ps )
h2::  |-  ( ph  ->  ch )
h3::  |-  ( ph  ->  th )
h4::  |-  ( ph  ->  ta )
h5::  |-  ( ps  ->  ( ch  ->  ( th  ->  ( ta  ->  et ) ) ) )
6:1,5:  |-  ( ph  ->  ( ch  ->  ( th  ->  ( ta  ->  et ) ) ) )
7:6:  |-  ( ch  ->  ( ph  ->  ( th  ->  ( ta  ->  et ) ) ) )
8:2,7:  |-  ( ph  ->  ( ph  ->  ( th  ->  ( ta  ->  et ) ) ) )
9:8:  |-  ( ph  ->  ( th  ->  ( ta  ->  et ) ) )
10:9:  |-  ( th  ->  ( ph  ->  ( ta  ->  et ) ) )
11:3,10:  |-  ( ph  ->  ( ph  ->  ( ta  ->  et ) ) )
12:11:  |-  ( ph  ->  ( ta  ->  et ) )
13:12:  |-  ( ta  ->  ( ph  ->  et ) )
14:4,13:  |-  ( ph  ->  ( ph  ->  et ) )
qed:14:  |-  ( ph  ->  et )
 |-  ( ph  ->  ps )   &    |-  ( ph  ->  ch )   &    |-  ( ph  ->  th )   &    |-  ( ph  ->  ta )   &    |-  ( ps  ->  ( ch  ->  ( th  ->  ( ta  ->  et )
 ) ) )   =>    |-  ( ph  ->  et )
 
Theorempm2.43bgbi 27316 Logical equivalence of a 2-left-nested implication and a 1-left-nested implicated when two antecedents of the former implication are identical. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.) The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown.
1::  |-  ( ( ph  ->  ( ps  ->  ( ph  ->  ch ) ) )  ->  ( ph  ->  ( ph  ->  ( ps  ->  ch ) ) ) )
2::  |-  ( ( ph  ->  ( ph  ->  ( ps  ->  ch ) ) )  ->  ( ph  ->  ( ps  ->  ch ) ) )
3:1,2:  |-  ( ( ph  ->  ( ps  ->  ( ph  ->  ch ) ) )  ->  ( ph  ->  ( ps  ->  ch ) ) )
4::  |-  ( ( ph  ->  ( ps  ->  ch ) )  ->  ( ps  ->  ( ph  ->  ch ) ) )
5:3,4:  |-  ( ( ph  ->  ( ps  ->  ( ph  ->  ch ) ) )  ->  ( ps  ->  ( ph  ->  ch ) ) )
6::  |-  ( ( ps  ->  ( ph  ->  ch ) )  ->  ( ph  ->  ( ps  ->  ( ph  ->  ch ) ) ) )
qed:5,6:  |-  ( ( ph  ->  ( ps  ->  ( ph  ->  ch ) ) )  <->  ( ps  ->  ( ph  ->  ch ) ) )
 |-  (
 ( ph  ->  ( ps 
 ->  ( ph  ->  ch )
 ) )  <->  ( ps  ->  (
 ph  ->  ch ) ) )
 
Theorempm2.43cbi 27317 Logical equivalence of a 3-left-nested implication and a 2-left-nested implicated when two antecedents of the former implication are identical. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.) The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown.
1::  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  )  ->  ( ph  ->  ( ps  ->  ( ph  ->  ( ch  ->  th ) ) ) ) )
2::  |-  ( ( ph  ->  ( ps  ->  ( ph  ->  ( ch  ->  th ) ) )  )  ->  ( ps  ->  ( ph  ->  ( ch  ->  th ) ) ) )
3:1,2:  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  )  ->  ( ps  ->  ( ph  ->  ( ch  ->  th ) ) ) )
4::  |-  ( ( ps  ->  ( ph  ->  ( ch  ->  th ) ) )  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
5:3,4:  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  )  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
6::  |-  ( ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  ->  ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) ) )
qed:5,6:  |-  ( ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) )  )  <->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
 |-  (
 ( ph  ->  ( ps 
 ->  ( ch  ->  ( ph  ->  th ) ) ) )  <->  ( ps  ->  ( ch  ->  ( ph  ->  th ) ) ) )
 
Theoremee233 27318 Non-virtual deduction form of e233 27574. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.) The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown.
h1::  |-  ( ph  ->  ( ps  ->  ch ) )
h2::  |-  ( ph  ->  ( ps  ->  ( th  ->  ta ) ) )
h3::  |-  ( ph  ->  ( ps  ->  ( th  ->  et ) ) )
h4::  |-  ( ch  ->  ( ta  ->  ( et  ->  ze ) ) )
5:1,4:  |-  ( ph  ->  ( ps  ->  ( ta  ->  ( et  ->  ze ) ) )  )
6:5:  |-  ( ta  ->  ( ph  ->  ( ps  ->  ( et  ->  ze ) ) )  )
7:2,6:  |-  ( ph  ->  ( ps  ->  ( th  ->  ( ph  ->  ( ps  ->  ( et  ->  ze ) ) ) ) ) )
8:7:  |-  ( ps  ->  ( th  ->  ( ph  ->  ( ps  ->  ( et  ->  ze ) ) ) ) )
9:8:  |-  ( th  ->  ( ph  ->  ( ps  ->  ( et  ->  ze ) ) )  )
10:9:  |-  ( ph  ->  ( ps  ->  ( th  ->  ( et  ->  ze ) ) )  )
11:10:  |-  ( et  ->  ( ph  ->  ( ps  ->  ( th  ->  ze ) ) )  )
12:3,11:  |-  ( ph  ->  ( ps  ->  ( th  ->  ( ph  ->  ( ps  ->  ( th  ->  ze ) ) ) ) ) )
13:12:  |-  ( ps  ->  ( th  ->  ( ph  ->  ( ps  ->  ( th  ->  ze ) ) ) ) )
14:13:  |-  ( th  ->  ( ph  ->  ( ps  ->  ( th  ->  ze ) ) )  )
qed:14:  |-  ( ph  ->  ( ps  ->  ( th  ->  ze ) ) )
 |-  ( ph  ->  ( ps  ->  ch ) )   &    |-  ( ph  ->  ( ps  ->  ( th  ->  ta ) ) )   &    |-  ( ph  ->  ( ps  ->  ( th  ->  et )
 ) )   &    |-  ( ch  ->  ( ta  ->  ( et  ->  ze ) ) )   =>    |-  ( ph  ->  ( ps  ->  ( th  ->  ze )
 ) )
 
Theoremimbi12 27319 Implication form of imbi12i 318. imbi12 27319 is imbi12VD 27683 without virtual deductions and was automatically derived from imbi12VD 27683 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  <->  ps )  ->  (
 ( ch  <->  th )  ->  (
 ( ph  ->  ch )  <->  ( ps  ->  th )
 ) ) )
 
Theoremimbi13 27320 Join three logical equivalences to form equivalence of implications. imbi13 27320 is imbi13VD 27684 without virtual deductions and was automatically derived from imbi13VD 27684 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  <->  ps )  ->  (
 ( ch  <->  th )  ->  (
 ( ta  <->  et )  ->  (
 ( ph  ->  ( ch 
 ->  ta ) )  <->  ( ps  ->  ( th  ->  et )
 ) ) ) ) )
 
Theoremee33 27321 Non-virtual deduction form of e33 27543. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.) The following User's Proof is a Virtual Deduction proof completed automatically by the tools program completeusersproof.cmd, which invokes Mel O'Cat's mmj2 and Norm Megill's Metamath Proof Assistant. The completed Virtual Deduction Proof (not shown) was minimized. The minimized proof is shown.
h1::  |-  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) )
h2::  |-  ( ph  ->  ( ps  ->  ( ch  ->  ta ) ) )
h3::  |-  ( th  ->  ( ta  ->  et ) )
4:1,3:  |-  ( ph  ->  ( ps  ->  ( ch  ->  ( ta  ->  et ) ) ) )
5:4:  |-  ( ta  ->  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) ) )
6:2,5:  |-  ( ph  ->  ( ps  ->  ( ch  ->  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) ) ) ) )
7:6:  |-  ( ps  ->  ( ch  ->  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) ) ) )
8:7:  |-  ( ch  ->  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) ) )
qed:8:  |-  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) )
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   &    |-  ( ph  ->  ( ps  ->  ( ch  ->  ta ) ) )   &    |-  ( th  ->  ( ta  ->  et ) )   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  et ) ) )
 
Theoremcon5 27322 Bi-conditional contraposition variation. This proof is con5VD 27710 automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  <->  -.  ps )  ->  ( -.  ph  ->  ps )
 )
 
Theoremcon5i 27323 Inference form of con5 27322. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph 
 <->  -.  ps )   =>    |-  ( -.  ph  ->  ps )
 
Theoremexlimexi 27324 Inference similar to Theorem 19.23 of [Margaris] p. 90. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ps  ->  A. x ps )   &    |-  ( E. x ph  ->  ( ph  ->  ps ) )   =>    |-  ( E. x ph 
 ->  ps )
 
Theoremsb5ALT 27325* Equivalence for substitution. Alternate proof of sb5 1994. This proof is sb5ALTVD 27723 automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( [ y  /  x ] ph  <->  E. x ( x  =  y  /\  ph )
 )
 
Theoremeexinst01 27326 exinst01 27431 without virtual deductions. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  E. x ps   &    |-  ( ph  ->  ( ps  ->  ch ) )   &    |-  ( ph  ->  A. x ph )   &    |-  ( ch  ->  A. x ch )   =>    |-  ( ph  ->  ch )
 
Theoremeexinst11 27327 exinst11 27432 without virtual deductions. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  E. x ps )   &    |-  ( ph  ->  ( ps  ->  ch ) )   &    |-  ( ph  ->  A. x ph )   &    |-  ( ch  ->  A. x ch )   =>    |-  ( ph  ->  ch )
 
Theoremvk15.4j 27328 Excercise 4j of Unit 15 of "Understanding Symbolic Logic", Fifth Edition (2008), by Virginia Klenk. This proof is the minimized Hilbert-style axiomatic version of the Fitch-style Natural Deduction proof found on page 442 of Klenk and was automatically derived from that proof. vk15.4j 27328 is vk15.4jVD 27724 automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  -.  ( E. x  -.  ph  /\ 
 E. x ( ps 
 /\  -.  ch )
 )   &    |-  ( A. x ch  ->  -.  E. x ( th  /\  ta )
 )   &    |- 
 -.  A. x ( ta 
 ->  ph )   =>    |-  ( -.  E. x  -.  th  ->  -.  A. x ps )
 
Theoremnotnot2ALT 27329 Converse of double negation. Alternate proof of notnot2 106. This proof is notnot2ALTVD 27725 automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( -.  -.  ph  ->  ph )
 
Theoremcon3ALT 27330 Contraposition. Alternate proof of con3 128. This proof is con3ALTVD 27726 automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  ->  ps )  ->  ( -.  ps  ->  -.  ph ) )
 
Theoremssralv2 27331* Quantification restricted to a subclass for two quantifiers. ssralv 3198 for two quantifiers. The proof of ssralv2 27331 was automatically generated by minimizing the automatically translated proof of ssralv2VD 27676. The automatic translation is by the tools program translatewithout_overwriting.cmd (Contributed by Alan Sare, 18-Feb-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( A  C_  B  /\  C  C_  D )  ->  ( A. x  e.  B  A. y  e.  D  ph  ->  A. x  e.  A  A. y  e.  C  ph ) )
 
Theoremsbc3org 27332 sbcorg 2997 with a 3-disjuncts. This proof is sbc3orgVD 27661 automatically translated and minimized. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 [. A  /  x ]. ( ph  \/  ps  \/  ch )  <->  ( [. A  /  x ]. ph  \/  [. A  /  x ]. ps  \/  [. A  /  x ].
 ch ) ) )
 
Theoremalrim3con13v 27333* Closed form of alrimi 1706 with 2 additional conjuncts having no occurences of the quantifying variable. This proof is 19.21a3con13vVD 27662 automatically translated and minimized. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  ->  A. x ph )  ->  ( ( ps  /\  ph  /\  ch )  ->  A. x ( ps 
 /\  ph  /\  ch )
 ) )
 
Theoremra4sbc2 27334* ra4sbc 3030 with two quantifying variables. This proof is ra4sbc2VD 27665 automatically translated and minimized. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  B  ->  ( C  e.  D  ->  (
 A. x  e.  B  A. y  e.  D  ph  -> 
 [. C  /  y ]. [. A  /  x ].
 ph ) ) )
 
Theoremsbcoreleleq 27335* Substitution of a set variable for another set variable in a 3-conjunct formula. Derived automatically from sbcoreleleqVD 27669. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 [. A  /  y ]. ( x  e.  y  \/  y  e.  x  \/  x  =  y
 ) 
 <->  ( x  e.  A  \/  A  e.  x  \/  x  =  A )
 ) )
 
Theoremtratrb 27336* If a class is transitive and any two distinct elements of the class are E-comparable, then every element of that class is transitive. Derived automatically from tratrbVD 27671. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( Tr  A  /\  A. x  e.  A  A. y  e.  A  ( x  e.  y  \/  y  e.  x  \/  x  =  y )  /\  B  e.  A ) 
 ->  Tr  B )
 
Theorem3ax5 27337 ax-5 1533 for a 3 element left-nested implication. Derived automatically from 3ax5VD 27672. (Contributed by Alan Sare, 31-Dec-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x ( ph  ->  ( ps  ->  ch )
 )  ->  ( A. x ph  ->  ( A. x ps  ->  A. x ch ) ) )
 
TheoremordelordALT 27338 An element of an ordinal class is ordinal. Proposition 7.6 of [TakeutiZaring] p. 36. This is an alternate proof of ordelord 4372 using the Axiom of Regularity indirectly through dford2 7275. dford2 is a weaker definition of ordinal number. Given the Axiom of Regularity, it need not be assumed that  _E  Fr  A because this is inferred by the Axiom of Regularity. ordelordALT 27338 is ordelordALTVD 27677 without virtual deductions and was automatically derived from ordelordALTVD 27677 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Feb-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( Ord  A  /\  B  e.  A )  ->  Ord  B )
 
Theoremsbcim2g 27339 Distribution of class substitution over a left-nested implication. Similar to sbcimg 2993. sbcim2g 27339 is sbcim2gVD 27685 without virtual deductions and was automatically derived from sbcim2gVD 27685 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 [. A  /  x ]. ( ph  ->  ( ps  ->  ch ) )  <->  ( [. A  /  x ]. ph  ->  (
 [. A  /  x ].
 ps  ->  [. A  /  x ].
 ch ) ) ) )
 
Theoremsbcbi 27340 Implication form of sbcbiiOLD 3008. sbcbi 27340 is sbcbiVD 27686 without virtual deductions and was automatically derived from sbcbiVD 27686 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 A. x ( ph  <->  ps )  ->  ( [. A  /  x ]. ph  <->  [. A  /  x ].
 ps ) ) )
 
Theoremtrsbc 27341* Formula-building inference rule for class substitution, substituting a class variable for the set variable of the transitivity predicate. trsbc 27341 is trsbcVD 27687 without virtual deductions and was automatically derived from trsbcVD 27687 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 [. A  /  x ].
 Tr  x  <->  Tr  A ) )
 
TheoremtruniALT 27342* The union of a class of transitive sets is transitive. Alternate proof of truni 4087. truniALT 27342 is truniALTVD 27688 without virtual deductions and was automatically derived from truniALTVD 27688 using the tools program translate..without..overwriting.cmd and Metamath's minimize command. (Contributed by Alan Sare, 18-Mar-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x  e.  A  Tr  x  ->  Tr  U. A )
 
Theoremsbcss 27343 Distribute proper substitution through a subclass relation. This theorem was automatically derived from sbcssVD 27693. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  B  ->  (
 [. A  /  x ]. C  C_  D  <->  [_ A  /  x ]_ C  C_  [_ A  /  x ]_ D ) )
 
TheoremonfrALTlem5 27344* Lemma for onfrALT 27351. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( [. ( a  i^i  x )  /  b ]. (
 ( b  C_  (
 a  i^i  x )  /\  b  =/=  (/) )  ->  E. y  e.  b  ( b  i^i  y )  =  (/) )  <->  ( ( ( a  i^i  x ) 
 C_  ( a  i^i 
 x )  /\  (
 a  i^i  x )  =/= 
 (/) )  ->  E. y  e.  ( a  i^i  x ) ( ( a  i^i  x )  i^i  y )  =  (/) ) )
 
TheoremonfrALTlem4 27345* Lemma for onfrALT 27351. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( [. y  /  x ]. ( x  e.  a  /\  ( a  i^i  x )  =  (/) )  <->  ( y  e.  a  /\  ( a  i^i  y )  =  (/) ) )
 
TheoremonfrALTlem3 27346* Lemma for onfrALT 27351. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( a  C_  On  /\  a  =/=  (/) )  ->  ( ( x  e.  a  /\  -.  (
 a  i^i  x )  =  (/) )  ->  E. y  e.  ( a  i^i  x ) ( ( a  i^i  x )  i^i  y )  =  (/) ) )
 
Theoremggen31 27347* gen31 27427 without virtual deductions. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  A. x th ) ) )
 
TheoremonfrALTlem2 27348* Lemma for onfrALT 27351. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( a  C_  On  /\  a  =/=  (/) )  ->  ( ( x  e.  a  /\  -.  (
 a  i^i  x )  =  (/) )  ->  E. y  e.  a  ( a  i^i  y )  =  (/) ) )
 
Theoremcbvexsv 27349* A theorem pertaining to the substitution for an existentially quantified variable when the substituted variable does not occur in the quantified wff. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( E. x ph  <->  E. y [ y  /  x ] ph )
 
TheoremonfrALTlem1 27350* Lemma for onfrALT 27351. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( a  C_  On  /\  a  =/=  (/) )  ->  ( ( x  e.  a  /\  ( a  i^i  x )  =  (/) )  ->  E. y  e.  a  ( a  i^i  y )  =  (/) ) )
 
TheoremonfrALT 27351 The epsilon relation is foundational on the class of ordinal numbers. onfrALT 27351 is an alternate proof of onfr 4389. onfrALTVD 27701 is the Virtual Deduction proof from which onfrALT 27351 is derived. The Virtual Deduction proof mirrors the working proof of onfr 4389 which is the main part of the proof of Theorem 7.12 of the first edition of TakeutiZaring. The proof of the corresponding Proposition 7.12 of [TakeutiZaring] p. 38 (second edition) does not contain the working proof equivalent of onfrALTVD 27701. This theorem does not rely on the Axiom of Regularity. (Contributed by Alan Sare, 22-Jul-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  _E  Fr  On
 
Theoremcsbeq2g 27352 Formula-building implication rule for class substitution. Closed form of csbeq2i 3068. csbeq2g 27352 is derived from the virtual deduction proof csbeq2gVD 27702. (Contributed by Alan Sare, 10-Nov-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A  e.  V  ->  (
 A. x  B  =  C  ->  [_ A  /  x ]_ B  =  [_ A  /  x ]_ C ) )
 
Theorem19.41rg 27353 Closed form of right-to-left implication of 19.41 1799, Theorem 19.41 of [Margaris] p. 90. Derived from 19.41rgVD 27712. (Contributed by Alan Sare, 8-Feb-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x ( ps  ->  A. x ps )  ->  ( ( E. x ph 
 /\  ps )  ->  E. x ( ph  /\  ps )
 ) )
 
Theoremopelopab4 27354* Ordered pair membership in a class abstraction of pairs. Compare to elopab 4230. (Contributed by Alan Sare, 8-Feb-2014.) (Revised by Mario Carneiro, 6-May-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( <. u ,  v >.  e. 
 { <. x ,  y >.  |  ph }  <->  E. x E. y
 ( ( x  =  u  /\  y  =  v )  /\  ph )
 )
 
Theorem2pm13.193 27355 pm13.193 26965 for two variables. pm13.193 26965 is Theorem *13.193 in [WhiteheadRussell] p. 179. Derived from 2pm13.193VD 27713. (Contributed by Alan Sare, 8-Feb-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ( x  =  u  /\  y  =  v )  /\  [ u  /  x ] [
 v  /  y ] ph )  <->  ( ( x  =  u  /\  y  =  v )  /\  ph )
 )
 
Theoremhbntal 27356 A closed form of hbn 1722. hbnt 1717 is another closed form of hbn 1722. (Contributed by Alan Sare, 8-Feb-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x ( ph  ->  A. x ph )  ->  A. x ( -.  ph  ->  A. x  -.  ph ) )
 
Theoremhbimpg 27357 A closed form of hbim 1723. Derived from hbimpgVD 27714. (Contributed by Alan Sare, 8-Feb-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( A. x ( ph  ->  A. x ph )  /\  A. x ( ps 
 ->  A. x ps )
 )  ->  A. x ( ( ph  ->  ps )  ->  A. x ( ph  ->  ps ) ) )
 
Theoremhbalg 27358 Closed form of hbal 1567. Derived from hbalgVD 27715. (Contributed by Alan Sare, 8-Feb-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. y ( ph  ->  A. x ph )  ->  A. y ( A. y ph  ->  A. x A. y ph ) )
 
Theoremhbexg 27359 Closed form of nfex 1733. Derived from hbexgVD 27716. (Contributed by Alan Sare, 8-Feb-2014.) (Revised by Mario Carneiro, 12-Dec-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x A. y (
 ph  ->  A. x ph )  ->  A. x A. y
 ( E. y ph  ->  A. x E. y ph ) )
 
Theorema9e2eq 27360* Alternate form of a9e 1817 for non-distinct  x,  y and  u  =  v. a9e2eq 27360 is derived from a9e2eqVD 27717. (Contributed by Alan Sare, 25-Mar-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x  x  =  y  ->  ( u  =  v  ->  E. x E. y ( x  =  u  /\  y  =  v ) ) )
 
Theorema9e2nd 27361* If at least two sets exist (dtru 4159) , then the same is true expressed in an alternate form similar to the form of a9e 1817. a9e2nd 27361 is derived from a9e2ndVD 27718. (Contributed by Alan Sare, 25-Mar-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( -.  A. x  x  =  y  ->  E. x E. y ( x  =  u  /\  y  =  v ) )
 
Theorema9e2ndeq 27362* "At least two sets exist" expressed in the form of dtru 4159 is logically equivalent to the same expressed in a form similar to a9e 1817 if dtru 4159 is false implies  u  =  v. a9e2ndeq 27362 is derived from a9e2ndeqVD 27719. (Contributed by Alan Sare, 25-Mar-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( -.  A. x  x  =  y  \/  u  =  v )  <->  E. x E. y ( x  =  u  /\  y  =  v )
 )
 
Theorem2sb5nd 27363* Equivalence for double substitution 2sb5 2075 without distinct  x,  y requirement. 2sb5nd 27363 is derived from 2sb5ndVD 27720. (Contributed by Alan Sare, 30-Apr-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( -.  A. x  x  =  y  \/  u  =  v )  ->  ( [ u  /  x ] [ v  /  y ] ph  <->  E. x E. y
 ( ( x  =  u  /\  y  =  v )  /\  ph )
 ) )
 
Theorem2uasbanh 27364* Distribute the unabbreviated form of proper substitution in and out of a conjunction. 2uasbanh 27364 is derived from 2uasbanhVD 27721. (Contributed by Alan Sare, 31-May-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ch 
 <->  ( E. x E. y ( ( x  =  u  /\  y  =  v )  /\  ph )  /\  E. x E. y
 ( ( x  =  u  /\  y  =  v )  /\  ps ) ) )   =>    |-  ( E. x E. y ( ( x  =  u  /\  y  =  v )  /\  ( ph  /\  ps ) )  <-> 
 ( E. x E. y ( ( x  =  u  /\  y  =  v )  /\  ph )  /\  E. x E. y
 ( ( x  =  u  /\  y  =  v )  /\  ps ) ) )
 
Theorem2uasban 27365* Distribute the unabbreviated form of proper substitution in and out of a conjunction. (Contributed by Alan Sare, 31-May-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( E. x E. y ( ( x  =  u 
 /\  y  =  v )  /\  ( ph  /\ 
 ps ) )  <->  ( E. x E. y ( ( x  =  u  /\  y  =  v )  /\  ph )  /\  E. x E. y
 ( ( x  =  u  /\  y  =  v )  /\  ps ) ) )
 
Theoreme2ebind 27366 Absorption of an existential quantifier of a double existential quantifier of non-distinct variables. e2ebind 27366 is derived from e2ebindVD 27722. (Contributed by Alan Sare, 27-Nov-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( A. x  x  =  y  ->  ( E. x E. y ph  <->  E. y ph )
 )
 
Theoremelpwgded 27367 elpwgdedVD 27727 in conventional notation. (Contributed by Alan Sare, 23-Apr-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  A  e.  _V )   &    |-  ( ps  ->  A  C_  B )   =>    |-  ( ( ph  /\  ps )  ->  A  e.  ~P B )
 
Theoremtrelded 27368 Deduction form of trel 4080. In a transitive class, the membership relation is transitive. (Contributed by Alan Sare, 3-Dec-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  Tr  A )   &    |-  ( ps  ->  B  e.  C )   &    |-  ( ch  ->  C  e.  A )   =>    |-  ( ( ph  /\  ps  /\ 
 ch )  ->  B  e.  A )
 
Theoremjaoded 27369 Deduction form of jao 500. Disjunction of antecedents. (Contributed by Alan Sare, 3-Dec-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ch ) )   &    |-  ( th  ->  ( ta  ->  ch )
 )   &    |-  ( et  ->  ( ps  \/  ta ) )   =>    |-  ( ( ph  /\  th  /\ 
 et )  ->  ch )
 
Theorem3imp31 27370 The importation inference 3imp 1150 with commutation of the first and third conjuncts of the assertion relative to the hypothesis. (Contributed by Alan Sare, 11-Sep-2016.) (Proof modification is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   =>    |-  ( ( ch  /\  ps 
 /\  ph )  ->  th )
 
Theorem3imp21 27371 The importation inference 3imp 1150 with commutation of the first and second conjuncts of the assertion relative to the hypothesis. (Contributed by Alan Sare, 11-Sep-2016.) (Proof modification is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   =>    |-  ( ( ps  /\  ph 
 /\  ch )  ->  th )
 
Theorembiimpa21 27372 biimpa 472 with commutation of the first and second conjuncts of the assertion. (Contributed by Alan Sare, 11-Sep-2016.)
 |-  ( ph  ->  ( ps  <->  ch ) )   =>    |-  ( ( ps 
 /\  ph )  ->  ch )
 
TheoremsbtT 27373 A substitution into a theorem remains true. sbt 1906 with the existence of no virtual hypotheses for the hypothesis expressed as the empty virtual hypothesis collection. (Contributed by Alan Sare, 4-Feb-2017.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (  T.  ->  ph )   =>    |- 
 [ y  /  x ] ph
 
16.22.2  What is Virtual Deduction?
 
Syntaxwvd1 27374 A Virtual Deduction proof in a Hilbert-style deductive system is the analog of a sequent calculus proof. A theorem is proven in a Gentzen system in order to prove more directly, which may be more intuitive and easier for some people. The analog of this proof in Metamath's Hilbert-style system is verified by the Metamath program.

Natural Deduction is a well known proof method orignally proposed by Gentzen in 1935 and comprehensively summarized by Prawitz in his 1965 monograph "Natural deduction: a proof-theoretic study". Gentzen wished to construct "a formalism that comes as close as possible to natural reasoning". Natural deduction is a response to dissatisfaction with axiomatic proofs such as Hilbert-style axiomatic proofs, which the proofs of Metamath are. In 1926, in Poland, Lukasiewicz advocated a more natural treatment of logic. Jaskowski made the earliest attempts at defining a more natural deduction. Natural deduction in its modern form was independently proposed by Gentzen.

Sequent calculus, the chief alternative to Natural Deduction, was created by Gentzen. The following is an except from Stephen Cole Kleene's seminal 1952 book "Introduction to Metamathematics", which contains the first formulation of sequent calculus in the modern style. Kleene states on page 440:

. . . the proof of (Gentzen's Hauptsatz) breaks down into a list of cases, each of which is simple to handle. . . . Gentzen's normal form for proofs in the predicate calculus requires a different classification of the deductive steps than is given by the postulates of the formal system of predicate calculus of Chapter IV (Section 19). The implication symbol  -> (the Metamath symbol for implication has been substituted here for the symbol used by Kleene) has to be separated in its role of mediating inferences from its role as a component symbol of the formula being proved. In the former role it will be replaced by a new formal symbol  ->.. (read "gives" or "entails"), to which properties will be assigned similar to those of the informal symbol  |- in our former derived rules.

Gentzen's classification of the deductive operations is made explicit by setting up a new formal system of the predicate calculus. The formal system of propositional and predicate calculus studied previously (Chapters IV ff.) we call now a "Hilbert-type system", and denote by H. Precisely, H denotes any one or a particular one of several systems, according to whether we are considering propositional calculus or predicate calculus, in the classical or the intuitionistic version (Section 23), and according to the sense in which we are using "term" and "formula" (Sections 117,25,31,37,72-76). The same respective choices will apply to the "Gentzen-type system G1" which we introduce now and the G2, G3 and G3a later.

The transformation or deductive rules of G1 will apply to objects which are not formulas of the system H, but are built from them by an additional formation rule, so we use a new term "sequent" for these objects. (Gentzen says "Sequenz", which we translate as "sequent", because we have already used "sequence" for any succession of objects, where the German is "Folge".) A sequent is a formal expression of the form  ph, . . . ,  ps  ->..  ch, . . . ,  th where  ph , . . . ,  ps and  ch, . . . ,  th are seqences of a finite number of 0 or more formulas (substituting Metamath notation for Kleene's notation). The part  ph, . . . ,  ps is the antecedent, and  ch, . . . ,  th the succedent of the sequent  ph, . . . ,  ps  ->..  ch, . . . ,  th.

When the antecedent and the succedent each have a finite number of 1 or more formulas, the sequent  ph, . . . ,  ps  ->..  ch, . . .  th has the same interpretation for G1 as the formula  ( ( ph  /\. . .  /\  ps )  ->  ( ch  \/. . .  \/  th ) ) for H. The interpretation extends to the case of an antecedent of 0 formulas by regarding  ( ph  /\. . .  /\ 
ps ) for 0 formulas (the "empty conjunction") as true and  ( ch  \/. . .  \/  th ) for 0 formulas (the "empty disjunction") as false.

. . . As in Chapter V, we use Greek capitals . . . to stand for finite sequences of zero or more formulas, but now also as antecedent (succedent), or parts of antecedent (succedent), with separating formal commas included. . . . (End of Kleene excerpt)

In chapter V entitled "Formal Deduction" Kleene states, on page 86:

Section 20. Formal deduction. Formal proofs of even quite elementary theorems tend to be long. As a price for having analyzed logical deduction into simple steps, more of those steps have to be used.

The purpose of formalizing a theory is to get an explicit definition of what constitutes proof in the theory. Having achieved this, there is no need always to appeal directly to the definition. The labor required to establish the formal provability of formulas can be greatly lessened by using metamathematical theorems concerning the existence of formal proofs. If the demonstrations of those theorems do have the finitary character which metamathematics is supposed to have, the demonstrations will indicate, at least implicitly, methods for obtaining the formal proofs. The use of the metamathematical theorems then amounts to abbreviation, often of very great extent, in the presentation of formal proofs.

The simpler of such metamathematical theorems we shall call derived rules, since they express principles which can be said to be derived from the postulated rules by showing that the use of them as additional methods of inference does not increase the class of provable formulas. We shall seek by means of derived rules to bring the methods for establishing the facts of formal provability as close as possible to the informal methods of the theory which is being formalized.

In setting up the formal system, proof was given the simplest possible structure, consisting of a single sequence of formulas. Some of our derived rules, called "direct rules", will serve to abbreviate for us whole segments of such a sequence; we can then, so to speak, use these segments as prefabricated units in building proofs.

But also, in mathematical practice, proofs are common which have a more complicated structure, employing "subsidiary deduction", i.e. deduction under assumptions for the sake of the argument, which assumptions are subsequently discharged. For example, subsidiary deduction is used in a proof by reductio ad absurdum, and less obtrusively when we place the hypothesis of a theorem on a par with proved propositions to deduce the conclusion. Other derived rules, called "subsidiary deduction rules", will give us this kind of procedure.

We now introduce, by a metamathematical definition, the notion of "formal deducibility under assumptions". Given a list  ph, . . .  ps of  0 or more (occurences of) formulas, a finite sequence of one or more (occurences of) formulas is called a (formal) deduction from the assumption formulas 
ph, . . .  ps, if each formula of the sequence is either one of the formulas  ph, . . .  ps, or an axiom, or an immediate consequence of preceding formulas of a sequence. A deduction is said to be deducible from the assumption formulas (in symbols,  ph,. . . . ,.  ps |-  ch), and is called the conclusion (or endformula) of the deduction. (The symbol  |- may be read "yields".) (End of Kleene excerpt)

Gentzen's normal form is a certain direct fashion for proofs and deductions. His sequent calculus, formulated in the modern style by Kleene, is the classical system G1. In this system, the new formal symbol  ->.. has properties similar to the informal symbol  |- of Kleene's above language of formal deducibility under assumptions.

Kleene states on page 440:

. . . This leads us to inquire whether there may not be a theorem about the predicate calculus asserting that, if a formula is provable (or deducible from other formulas), it is provable (or deducible) in a certain direct fashion; in other words, a theorem giving a normal form for proofs and deductions, the proofs and deduction in normal form being in some sense direct. (End of Kleene excerpt)

There is such a theorem, which was proven by Kleene.

Formal proofs in H of even quite elementary theorems tend to be long. As a price for having analyzed logical deduction into simple steps, more of those steps have to be used. The proofs of Metamath are fully detailed formal proofs. We wish to have a means of proving Metamath theorems and deductions in a more direct fashion. Natural Deduction is a system for proving theorems and deductions in a more direct fashion. However, Natural Deduction is not compatible for use with Metamath, which uses a Hilbert-type system. Instead, Kleene's classical system G1 may be used for proving Metamath deductions and theorems in a more direct fashion.

The system of Metamath is an H system, not a Gentzen system. Therefore, proofs in Kleene's classical system G1 ("G1") cannot be included in Metamath's system H, which we shall henceforth call "system H" or "H". However, we may translate proofs in G1 into proofs in H.

By Kleene's THEOREM 47 (page 446)

if  |-  ->..  ph in G1 then  |-  ph in H

By Kleene's COROLLARY of THEOREM 47 (page 448)

if  |-  ph  ->..  ps in G1 then  |-  (. ph  ->.  ps ). in H
if  |-  ph ,. ps  ->..  ch in G1 then  |-  (. (. ph ,. ps ).  ->.  ch ). in H
if  |-  ph ,. ps ,. ch  ->..  th in G1 then  |-  (. (. ph ,. ps ,. ch ).  ->.  th ). in H

 ->. denotes the same connective denoted by  ->. " , " , in the context of Virtual Deduction, denotes the same connective denoted by  /\. This Virtual Deduction notation is specified by the following set.mm definitions:

df-vd1 27375  |-  ( (. ph  ->.  ps ).  <->  ( ph  ->  ps ) )
dfvd2an 27388  |-  ( (. (. ph ,. ps ).  ->.  ch ).  <->  ( ( ph  /\  ps )  ->  ch ) )
dfvd3an 27400  |-  ( (. (. ph ,. ps ,. ch ).  ->.  th ).  <->  ( ( ph  /\  ps  /\  ch )  ->  th ) )

 ->. replaces 
->.. in the analog in H of a sequent in G1 having a non-empty antecedent. If  ->. occurs as the outermost connective denoted by 
->. or  -> and occurs exactly once, we call the analog in H of a sequent in G1 a "virtual deduction" because the corresponding  ->.. of the sequent is assigned properties similar to  |-.

While sequent calculus proofs (proofs in G1) may have as steps sequents with 0, 1, or more formulas in the succedent, we shall only prove in G1 using sequents with exactly 1 formula in the succedent.

The User proves in G1 in order to obtain the benefits of more direct proving using sequent calculus, then translates the proof in G1 into a proof in H. The reference theorems and deductions to be used for proving in G1 are translations of theorems and deductions in set.mm.

Each theorem  |-  ph in set.mm corresponds to the theorem  |-  ->..  ph in G1. Deductions in G1 corresponding to deductions in H are similarly determined. Theorems in H with one or more occurences of either 
->. or  -> may also be translated into theorems in G1 for by replacing the outermost occurence of  ->. or  -> of the theorem in H with  ->... Deductions in H may be translated into deductions in G1 in a similar manner. The only theorems and deductions in H useful for proving in G1 for the purpose of obtaining proofs in H are those in which, for each hypothesis or assertion, there are 0 or 1 occurences of  ->. and it is the outermost occurence of  ->. or  ->. Kleene's THEOREM 46 and its COROLLARY 2 are used for translating from H to G1. By Kleene's THEOREM 46 (page 445)

if  |-  ph in H then  |-  ->..  ph in G1

By Kleene's COROLLARY 2 of THEOREM 46 (page 446)

if  |-  (. ph  ->.  ps ). in H then  |-  ph  ->..  ps in G1
if  |-  (. (. ph ,. ps ).  ->.  ch ). in H then  |-  ph ,. ps  ->..  ch in G1
if  |-  (. (. ph ,. ps ,. ch ).  ->.  th ). in H then  |-  ph ,. ps ,. ch  ->..  th in G1

The procedure for more direct proving of theorems or deductions in H is as follows. The User proves in G1. He(she) uses translated set.mm theorems and deductions as reference theorems and deductions. His(her) proof is only a guess in the sense that he(she) can't verify his(her) proof in G1 because he(she) doesn't have an automated proof checker to use. The proof in G1 is translated into its analog in H for verification by the Metamath program. This proof is called the Virtual Deduction proof. This proof may then be translated into a conventional Metamath proof automatically, removing the unnecessary Virtual Deduction symbols.

The translations from H to G1 and G1 to H are trivial. In practice, they may be done without much thought. In principle, they must be done, because the proving is done using sequents, which do not exist in H.

The analogs in H of the postulates of G1 are the set.mm postulates. The postulates in G1 corresponding to the Metamath postulates are not the classical system G1 postulates of Kleene (pages 442 and 443). set.mm has the predicate calculus postulates and other posulates. The Kleene classical system G1 postulates correspond to predicate calculus postulates which differ from the Metamath system G1 postulates corresponding to the predicate calculus postulates of Metamath's system H. Metamath's predicate calculus G1 postulates are presumably deducible from the Kleene classical G1 postulates and the Kleene classical G1 postulates are deducible from Metamath's G1 postulates. It should be recognized that, because of the different postulates, the classical G1 system corresponding to Metamath's system H is not identical to Kleene's classical system G1.

Why not create a separate database (setg.mm) of proofs in G1, avoiding the need to translate from H to G1 and from G1 back to H? The translations are trivial. Sequents make the language more complex than is necessary. More direct proving using sequent calculus may be done as a means towards the end of constructing proofs in H. Then, the language may be kept as simple as possible. A system G1 database would be redundant because it would duplicate the information contained in the corresponding system H database.

For earlier proofs, each "User's Proof" in the web page description of a Virtual Deduction proof in set.mm is the analog in H of the User's working proof in G1. The User's Proof is automatically completed by completeusersproof.cmd. The completed proof is the Virtual Deduction proof, which is the analog in H of the corresponding fully detailed proof in G1. The completed Virtual Deduction proof of these earlier proofs may be automatically translated into a conventional Metamath proof.

In September of 2016 completeusersproof.c was released. The input for completeusersproof.c is a Virtual Deduction User's Proof. Unlike completeusersproof.cmd, the completed proof is in conventional notation. completeusersproof.c eliminates the virtual deduction notation of the User's Proof after utilizing the information it provides.

Applying mmj2's unify command is essential to completeusersproof.c. The mmj2 program is invoked within the completeusersproof.c function mmj2Unify(). The original mmj2 program was written by Mel O'Cat. Mario Carneiro has enhanced it. mmj2Unify() is called multiple times during the execution of completeusersproof.

A Virtual Deduction proof is the Metamath-specific version of a Natural Deduction Proof. A Virtual Deduction proof generally cannot be directly input on a mmj2 Proof Worksheet and completed by the mmj2 tool because it is usually missing some technical proof steps which are not part of the Virtual Deduction proof but are necessary for a complete Metamath Proof. These missing technical steps may be automatically added by an automated proof assistant. completeusersproof.c is such a proof assistant. completeusersproof.c adds the missing technical steps and finds the reference theorems and deductions in set.mm which unify with the subproofs of the proof.

The User may write a Virtual Deduction proof and automatically transform it into a complete Metamath proof using the completeusersproof tool. The completed proof has been checked by the Metamath program. The task of writing a complete Metamath proof is reduced to writing what is essentially a Natural Deduction Proof.

Generally, proving using Virtual Deduction and completeusersproof reduces the amount Metamath-specific knowledge required by the User. Often, no knowledge of the specific theorems and deductions in set.mm is required to write some of the subproofs of a Virtual Deduction proof. Often, no knowledge of the Metamath-specific names of reference theorems and deductions in set.mm is required for writing some of the subproofs of a User's Proof. Often, the User may write subproofs of a proof using theorems or deductions commonly used in mathematics and correctly assume that some form of each is contained in set.mm and that completeusersproof will automatically generate the technical steps necessary to utilize them to complete the subproofs. Often, the fraction of the work which may be considered tedious is reduced and the total amount of work is reduced.

 wff  (.
 ph 
 ->.  ps ).
 
16.22.3  Virtual Deduction Theorems
 
Definitiondf-vd1 27375 Definition of virtual deduction. (Contributed by Alan Sare, 21-Apr-2011.) (New usage is discouraged.)
 |-  ( (. ph  ->.  ps ).  <->  ( ph  ->  ps ) )
 
Theoremin1 27376 Inference form of df-vd1 27375. Virtual deduction introduction rule of converting the virtual hypothesis of a 1-virtual hypothesis virtual deduction into an antecedent. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (. ph  ->.  ps
 ).   =>    |-  ( ph  ->  ps )
 
Theoremiin1 27377 in1 27376 without virtual deductions. (Contributed by Alan Sare, 23-Jul-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ps )   =>    |-  ( ph  ->  ps )
 
Theoremdfvd1ir 27378 Inference form of df-vd1 27375 with the virtual deduction as the assertion. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ps )   =>    |- 
 (. ph  ->.  ps ).
 
Theoremidn1 27379 Virtual deduction identity rule which is id 21 with virtual deduction symbols. (Contributed by Alan Sare, 24-Jun-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (. ph  ->.  ph ).
 
Theoremax172 27380* Quantification of two variables over a formula in which they do not occur. (Contributed by Alan Sare, 12-Apr-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  A. x A. y ph )
 
Theoremdfvd1imp 27381 Left-to-right part of definition of virtual deduction. (Contributed by Alan Sare, 21-Apr-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( (. ph  ->.  ps ).  ->  ( ph  ->  ps ) )
 
Theoremdfvd1impr 27382 Right-to-left part of definition of virtual deduction. (Contributed by Alan Sare, 21-Apr-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  ->  ps )  ->  (. ph  ->.  ps ). )
 
Syntaxwvd2 27383 Syntax for a 2-hypothesis virtual deduction. (New usage is discouraged.)
 wff  (.
 ph ,. ps  ->.  ch ).
 
Definitiondf-vd2 27384 Definition of a 2-hypothesis virtual deduction. (Contributed by Alan Sare, 14-Nov-2011.) (New usage is discouraged.)
 |-  ( (. ph ,. ps  ->.  ch ).  <->  ( ( ph  /\ 
 ps )  ->  ch )
 )
 
Theoremdfvd2 27385 Definition of a 2-hypothesis virtual deduction. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( (. ph ,. ps  ->.  ch ).  <->  ( ph  ->  ( ps  ->  ch )
 ) )
 
Syntaxwvhc2 27386 Syntax for a 2-virtual hypotheses collection. (Contributed by Alan Sare, 23-Apr-2015.) (New usage is discouraged.)
 wff  (.
 ph ,. ps ).
 
Definitiondf-vhc2 27387 Definition of a 2-virtual hypotheses collection. (Contributed by Alan Sare, 23-Apr-2015.) (New usage is discouraged.)
 |-  ( (. ph ,. ps ).  <->  (
 ph  /\  ps )
 )
 
Theoremdfvd2an 27388 Definition of a 2-hypothesis virtual deduction in vd conjunction form. (Contributed by Alan Sare, 23-Apr-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( (. (. ph ,. ps ).  ->.  ch ).  <->  ( ( ph  /\ 
 ps )  ->  ch )
 )
 
Theoremdfvd2ani 27389 Inference form of dfvd2an 27388. (Contributed by Alan Sare, 23-Apr-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (. (. ph
 ,. ps ).  ->.  ch ).   =>    |-  ( ( ph  /\ 
 ps )  ->  ch )
 
Theoremdfvd2anir 27390 Right-to-left inference form of dfvd2an 27388. (Contributed by Alan Sare, 23-Apr-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (
 ( ph  /\  ps )  ->  ch )   =>    |- 
 (. (. ph ,. ps ).  ->.  ch ).
 
Theoremdfvd2i 27391 Inference form of dfvd2 27385. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (. ph ,. ps  ->.  ch ).   =>    |-  ( ph  ->  ( ps  ->  ch ) )
 
Theoremdfvd2ir 27392 Right-to-left inference form of dfvd2 27385. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ch ) )   =>    |- 
 (. ph ,. ps  ->.  ch ).
 
Syntaxwvd3 27393 Syntax for a 3-hypothesis virtual deduction. (New usage is discouraged.)
 wff  (.
 ph ,. ps ,. ch  ->.  th
 ).
 
Syntaxwvhc3 27394 Syntax for a 3-virtual hypotheses collection. (Contributed by Alan Sare, 13-Jun-2015.) (New usage is discouraged.)
 wff  (.
 ph ,. ps ,. ch ).
 
Definitiondf-vhc3 27395 Definition of a 3-virtual hypotheses collection. (Contributed by Alan Sare, 13-Jun-2015.) (New usage is discouraged.)
 |-  ( (. ph ,. ps ,. ch ).  <->  ( ph  /\  ps  /\ 
 ch ) )
 
Definitiondf-vd3 27396 Definition of a 3-hypothesis virtual deduction. (Contributed by Alan Sare, 14-Nov-2011.) (New usage is discouraged.)
 |-  ( (. ph ,. ps ,. ch  ->. 
 th ).  <->  ( ( ph  /\ 
 ps  /\  ch )  ->  th ) )
 
Theoremdfvd3 27397 Definition of a 3-hypothesis virtual deduction. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( (. ph ,. ps ,. ch  ->. 
 th ).  <->  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) ) )
 
Theoremdfvd3i 27398 Inference form of dfvd3 27397. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  (. ph ,. ps ,. ch  ->.  th ).   =>    |-  ( ph  ->  ( ps  ->  ( ch  ->  th ) ) )
 
Theoremdfvd3ir 27399 Right-to-left inference form of dfvd3 27397. (Contributed by Alan Sare, 14-Nov-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( ph  ->  ( ps  ->  ( ch  ->  th )
 ) )   =>    |- 
 (. ph ,. ps ,. ch  ->. 
 th ).
 
Theoremdfvd3an 27400 Definition of a 3-hypothesis virtual deduction in vd conjunction form. (Contributed by Alan Sare, 13-Jun-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( (. (. ph ,. ps ,. ch ).  ->.  th ).  <->  ( ( ph  /\ 
 ps  /\  ch )  ->  th ) )
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