new_contents: 
  def gcd(m, n):
    """greatest common denominator, with euclid"""
    if n > m:
      m, n = n, m
  
    r = m % n # get the remainder
  
    if r == 0:
      return n
  
    return gcd(n, r)
  
  
  # testing
  
  print gcd(100, 90) # 10
  print gcd(24, 18) # 6
  print gcd(55, 35) # 5
  print gcd(1000,999) # 1
  
  
subject: add common algorithms