(VAR f x y )
(RULES 
        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))
        
)