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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 is_subdirectory(path_a, path_b):
 """Returns True if `path_a` is a subdirectory of `path_b`."""

    path_a = os.path.realpath(path_a)
    path_b = os.path.realpath(path_b)
    relative = os.path.relpath(path_a, path_b)
    return (not relative.startswith(os.pardir + os.sep))