(VAR p x xs y ys z )
(RULES 
        if(true, xs, ys) -> xs
        if(false, xs, ys) -> ys
        app(nil, xs) -> xs
        app(cons(x, xs), ys) -> cons(x, app(xs, ys))
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        gr(0, y) -> false
        gr(s(x), 0) -> true
        gr(s(x), s(y)) -> gr(x, y)
        filter(p, nil) -> nil
        filter(p, cons(x, xs)) -> if(, cons(x, filter(p, xs)), filter(p, xs))
        qsort(nil) -> nil
        qsort(cons(x, xs)) -> app(qsort(filter(, xs)), app(cons(x, nil), qsort(filter(, xs))))
        
)