def common_prefix(s1, s2):
    """ Return prefix common of 2 strings """
    if not s1 or not s2:
        return ""
    k = 0
    while s1[k] == s2[k]:
        k = k + 1
        if k >= len(s1) or k >= len(s2):
            return s1[0:k]
    return s1[0:k]