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)))