(VAR u x y z )
(RULES 
        -(x, 0) -> x
        -(s(x), s(y)) -> -(x, y)
        <=(0, y) -> true
        <=(s(x), 0) -> false
        <=(s(x), s(y)) -> <=(x, y)
        if(true, x, y) -> x
        if(false, x, y) -> y
        perfectp(0) -> false
        perfectp(s(x)) -> f(x, s(0), s(x), s(x))
        f(0, y, 0, u) -> true
        f(0, y, s(z), u) -> false
        f(s(x), 0, z, u) -> f(x, u, -(z, s(x)), u)
        f(s(x), s(y), z, u) -> if(<=(x, y), f(s(x), -(y, x), z, u), f(x, u, z, u))
        
)