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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 are_equivalent(a_xml, b_xml, encoding=None):
 """Return True if two XML docs are semantically equivalent, else False. - TODO: Include test for tails. Skipped for now because tails are not used in any D1 types. """

    assert isinstance(a_xml, str)
    assert isinstance(b_xml, str)
    a_tree = str_to_etree(a_xml, encoding)
    b_tree = str_to_etree(b_xml, encoding)
    return are_equal_or_superset(a_tree, b_tree) and are_equal_or_superset(
        b_tree, a_tree
    )