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