The Simplification of the Induction Conclusion (Step 0)
Subgoal *1/2' (IMPLIES (AND (CONSP A) (EQUAL (APP (APP (CDR A) B) C) (APP (CDR A) (APP B C)))) (EQUAL (APP (APP A B) C) (APP A (APP B C)))).
Click on the link above to replace (APP A B) by its definition.