code: def union(self, other, renorm=True):
          
          # merge the pixels that are common to both
          for d in range(1, min(self.maxdepth, other.maxdepth)+1):
              self.add_pixels(other.pixeldict[d], d)
  
          # if the other region is at higher resolution, then include a degraded version of the remaining pixels.
          if self.maxdepth < other.maxdepth:
              for d in range(self.maxdepth+1, other.maxdepth+1):
                  for p in other.pixeldict[d]:
                      # promote this pixel to self.maxdepth
                      pp = p/4**(d-self.maxdepth)
                      self.pixeldict[self.maxdepth].add(pp)
          if renorm:
              self._renorm()
          return
docs: Add another Region by performing union on their pixlists.
  
          Parameters
          ----------
          other : :class:`AegeanTools.regions.Region`
              The region to be combined.
  
          renorm : bool
              Perform renormalisation after the operation?
              Default = True.