new_contents: def gcd(a, b, res):
  	if a == b:
  		return res * a
  	elif (a % 2 == 0) and (b % 2 == 0):
  		return gcd(a // 2, b // 2, 2 * res)
  	elif (a % 2 == 0):
  		return gcd(a // 2, b, res)
  	elif (b % 2 == 0):
  		return gcd(a, b // 2, res)
  	elif a > b:
  		return gcd(a - b, b, res)
  	else:
  		return gcd(a, b - a, res)
  
  print(gcd(258, 321, 1))
  
subject: Create BinaryGCD.py (#549)