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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)