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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 union(self, other, sort=None):
 """ Form the union of two MultiIndex objects Parameters other : MultiIndex or array / Index of tuples sort : False or None, default None Whether to sort the resulting Index. * None : Sort the result, except when 1. `self` and `other` are equal. 2. `self` has length 0. 3. Some values in `self` or `other` cannot be compared. A RuntimeWarning is issued in this case. * False : do not sort the result. .. versionadded:: 0.24.0 .. versionchanged:: 0.24.1 Changed the default value from ``True`` to ``None`` (without change in behaviour). Returns ------- Index """

        self._validate_sort_keyword(sort)
        self._assert_can_do_setop(other)
        other, result_names = self._convert_can_do_setop(other)

        if len(other) == 0 or self.equals(other):
            return self

        # TODO: Index.union returns other when `len(self)` is 0.

        uniq_tuples = lib.fast_unique_multiple([self._ndarray_values,
                                                other._ndarray_values],
                                               sort=sort)

        return MultiIndex.from_arrays(lzip(*uniq_tuples), sortorder=0,
                                      names=result_names)