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