(VAR f x y z )
(RULES 
        app(app(app(fold,f),x),nil) -> x
        app(app(app(fold,f),x),app(app(cons,y),z)) -> app(app(f,y),app(app(app(fold,f),x),z))
        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)
        sum -> app(app(fold,add),0)
        prod -> app(app(fold,mul),app(s,0))
        
)