Metamath Proof Explorer

Theorem bj-bialal

Description: When ph is substituted for ps , both sides express a form of nonfreeness. (Contributed by BJ, 20-Oct-2019)

Ref Expression
Assertion bj-bialal 
|- ( A. x ( A. x ph -> ps ) <-> ( A. x ph -> A. x ps ) )

Proof

Step Hyp Ref Expression
1 nfa1 
 |-  F/ x A. x ph
2 1 19.21 
 |-  ( A. x ( A. x ph -> ps ) <-> ( A. x ph -> A. x ps ) )