(VAR x y)
(RULES
 -(x, 0) -> x
 -(s(x), s(y)) -> -(x, y) 
 p(s(x)) -> x
 f(s(x), y) -> f(p(-(s(x), y)), p(-(y, s(x))))
 f(x, s(y)) -> f(p(-(x, s(y))), p(-(s(y), x)))
)