code: def union(self, another_is):
          
          result = IntervalSet()
          if another_is.empty():
              result._intervals = self._intervals
          elif self.empty():
              result._intervals = another_is._intervals
          else:
              # res has no overlapping intervals
              result._intervals = IntervalSet.merge(self._intervals,
                                                    another_is._intervals,
                                                    lambda in_a, in_b: in_a or in_b)
          return result
docs: Return the union between self and ``another_is``.
  
          Parameters
          ----------
          another_is : `IntervalSet`
              an IntervalSet object.
          Returns
          -------
          interval : `IntervalSet`
              the union of self with ``another_is``.