Subgoal *1/1
(IMPLIES (ENDP A)
(EQUAL (APP (APP A B) C)
(APP A (APP B C)))).
By the simple :definition ENDP we reduce the conjecture to
Subgoal *1/1'
(IMPLIES (NOT (CONSP A))
(EQUAL (APP (APP A B) C)
(APP A (APP B C)))).
But simplification reduces this to T, using the :definition APP and
primitive type reasoning.