testgcd = (f,g)->(
      h:=f;
      s:=g;
      while s!=0 do (
           rem:= h%s;
	   h =s;
	   s =rem );
	return h)
      
      
testgcd(3,4)       
testgcd(12,18)


R=ZZ/101[x]
f=(x^2+1)^3*(x^20+x^3+35*x+1)
g=(x^2+1)^2*(x^21+x^5+3*x+21)
testgcd(f,g)
gcd(f,g)


R=QQ[x]
f=(x^2+1)^3*(x^20+x^3+35*x+1)
g=(x^2+1)^2*(x^21+x^5+3*x+21)
testgcd(f,g)
gcd(f,g)