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

Click on the link above to use the Induction Hypothesis (which is the
second of the two hypotheses above and which is identical to the rewritten
conclusion).