MAYBE

Problem:
 app(app(add(),0()),y) -> y
 app(app(add(),app(s(),x)),y) -> app(s(),app(app(add(),x),y))
 app(app(mult(),0()),y) -> 0()
 app(app(mult(),app(s(),x)),y) -> app(app(add(),app(app(mult(),x),y)),y)
 app(app(app(rec(),f),x),0()) -> x
 app(app(app(rec(),f),x),app(s(),y)) -> app(app(f,app(s(),y)),app(app(app(rec(),f),x),y))
 fact() -> app(app(rec(),mult()),app(s(),0()))

Proof:
 Open