code: def hamming_distance(s1, s2):
      
      if len(s1) != len(s2):
          raise ValueError("Undefined for sequences of unequal length")
      return sum(ch1 != ch2 for ch1, ch2 in zip(s1, s2))
docs: Return the Hamming distance between equal-length sequences
      (http://en.wikipedia.org/wiki/Hamming_distance )