<-0Marked_gcdifsgcdzyxgcd(x,0)xgcd(0,y)ygcd(s(x),s(y))if(<(x,y),gcd(s(x),-(y,x)),gcd(-(x,y),s(y)))gcd(x,0)xgcd(0,y)ygcd(s(x),s(y))if(<(x,y),gcd(s(x),-(y,x)),gcd(-(x,y),s(y)))Marked_gcd(s(x),s(y))Marked_gcd(s(x),-(y,x))Marked_gcd(s(x),s(y))Marked_gcd(-(x,y),s(y))