(from AG01 3.6b)
(VAR x y)
(RULES
le(0,y) -> true
le(s(x),0) -> false
le(s(x),s(y)) -> le(x,y)
minus(0,y)  -> 0
minus(s(x),y) -> if_minus(le(s(x),y),s(x),y)
if_minus(true,s(x),y) -> 0
if_minus(false,s(x),y) -> s(minus(x,y))
gcd(0,y) -> y
gcd(s(x),0) -> s(x)
gcd(s(x),s(y)) -> if_gcd(le(y,x),s(x),s(y))
if_gcd(true,s(x),s(y)) -> gcd(minus(x,y),s(y))
if_gcd(false,s(x),s(y)) -> gcd(minus(y,x),s(x)))