(VAR x y)
(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(s(x),s(y)) -> gcd(-(s(max(x,y)), s(min(x,y))), s(min(x,y)))
  gcd(s(x),0) -> s(x)
  gcd(0,s(y)) -> s(y)
)
(COMMENT gdc, extended version of 'secret2005 - tpa1')