(VAR x y z )
(RULES 
        min(x,0) -> 0
        min(0,y) -> 0
        min(s(x),s(y)) -> s(min(x,y))
        max(x,0) -> x
        max(0,y) -> y
        max(s(x),s(y)) -> s(max(x,y))
        -(x,0) -> x
        -(s(x),s(y)) -> -(x,y)
        gcd(nil) -> 0
        gcd(cons(x,nil)) -> x
        gcd(cons(0,y)) -> gcd(y)
        gcd(cons(x,cons(y,z))) -> gcd(cons(-(x,y),cons(y,z)))
        gcd(cons(x,cons(y,z))) ->= gcd(cons(max(x,y),cons(min(x,y),z)))
)