def common_substring(s1, s2):
    """
    Returns the longest common substring to the two strings, starting from the
    left.
    """
    chunks = []
    path1 = splitall(s1)
    path2 = splitall(s2)
    for (dir1, dir2) in zip(path1, path2):
        if dir1 != dir2:
            break
        chunks.append(dir1)
    return os.path.join(*chunks)
Returns the longest common substring to the two strings, starting from the
    left.