The Expansion of ENDP in the Induction Step (Step 1)

Subgoal *1/2
(IMPLIES (AND (NOT (NOT (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)))).

The bold text is the instantiated definition of ENDP.

Now click on the link above to simplify (NOT (NOT (CONSP A)))