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