docs: 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
      )