(VAR f g x y )
(RULES 
        app(app(plus,0),y) -> y
        app(app(plus,app(s,x)),y) -> app(s,app(app(plus,x),y))
        app(app(times,0),y) -> 0
        app(app(times,app(s,x)),y) -> app(app(plus,app(app(times,x),y)),y)
        app(app(app(comp,f),g),x) -> app(f,app(g,x))
        app(twice,f) -> app(app(comp,f),f)
        
)