Subgoal *1/2'
(IMPLIES (AND (CONSP A)
(EQUAL (APP (APP (CDR A) B) C)
(APP (CDR A) (APP B C))))
(EQUAL (CONS (CAR A)
(APP (APP (CDR A) B)
C))
(CONS (CAR A)
(APP (CDR A) (APP B C))))).

Click on the link above to apply the Axiom that
(EQUAL (CONS x y) (CONS u v)) is equal to the conjunction of
(EQUAL x u) and (EQUAL y v). In this case, (EQUAL x u)
is trivial, (EQUAL (CAR A) (CAR A)).