Subgoal *1/2'
(IMPLIES (AND (CONSP A)
(EQUAL (APP (APP (CDR A) B) C)
(APP (CDR A) (APP B C))))
(EQUAL (IF T
(CONS (CAR A)
(APP (CDR (CONS (CAR A) (APP (CDR A) B)))
C))
C)
(APP A (APP B C)))).

Click on the link above to apply the Axiom (CDR (CONS x y)) = y.