Theorem ifnmfalse 42504

Description: If A is not a member of B, but an "if" condition requires it, then the "false" branch results. This is a simple utility to provide a slight shortening and simplification of proofs vs. applying iffalse 4095 directly in this case. (Contributed by David A. Wheeler, 15-May-2015.)

Assertion

Ref	Expression
ifnmfalse	|- ( A e/ B -> if ( A e. B , C , D ) = D )

Proof of Theorem ifnmfalse

Step	Hyp	Ref	Expression
1		df-nel 2898	. 2 |- ( A e/ B <-> -. A e. B )
2		iffalse 4095	. 2 |- ( -. A e. B -> if ( A e. B , C , D ) = D )
3	1, 2	sylbi 207	1 |- ( A e/ B -> if ( A e. B , C , D ) = D )

Syntax hints:   -. wn 3   -> wi 4   = wceq 1483   e. wcel 1990   e/ wnel 2897   ifcif 4086

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1722  ax-4 1737  ax-5 1839  ax-6 1888  ax-7 1935  ax-9 1999  ax-10 2019  ax-11 2034  ax-12 2047  ax-13 2246  ax-ext 2602

This theorem depends on definitions:  df-bi 197  df-or 385  df-an 386  df-tru 1486  df-ex 1705  df-nf 1710  df-sb 1881  df-clab 2609  df-cleq 2615  df-clel 2618  df-nel 2898  df-if 4087

This theorem is referenced by: (None)