(VAR u x y z )
(RULES 
        minus(0, y) -> 0
        minus(s(x), 0) -> s(x)
        minus(s(x), s(y)) -> minus(x, y)
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(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, minus(z, s(x)), u)
        f(s(x), s(y), z, u) -> if(le(x, y), f(s(x), minus(y, x), z, u), f(x, u, z, u))
        
)