(VAR x y) (RULES -(x, 0) -> x -(0, s(y)) -> 0 -(s(x), s(y)) -> -(x, y) f(0) -> 0 f(s(x)) -> -(s(x), g(f(x))) g(0) -> s(0) g(s(x)) -> -(s(x), f(g(x))) ) (COMMENT Example 8 in \cite{G96}. )