(VAR f x xs y )
(RULES 
        app(app(app(if,true),x),y) -> x
        app(app(app(if,false),x),y) -> y
        app(app(filter,f),nil) -> nil
        app(app(filter,f),app(app(cons,x),xs)) -> app(app(app(if,app(f,x)),app(app(cons,x),app(app(filter,f),xs))),app(app(filter,f),xs))
        
)