code: def _renorm(self):
          
          self.demoted = set()
          # convert all to lowest level
          self._demote_all()
          # now promote as needed
          for d in range(self.maxdepth, 2, -1):
              plist = self.pixeldict[d].copy()
              for p in plist:
                  if p % 4 == 0:
                      nset = set((p, p+1, p+2, p+3))
                      if p+1 in plist and p+2 in plist and p+3 in plist:
                          # remove the four pixels from this level
                          self.pixeldict[d].difference_update(nset)
                          # add a new pixel to the next level up
                          self.pixeldict[d-1].add(p/4)
          self.demoted = set()
          return
docs: Remake the pixel dictionary, merging groups of pixels at level N into a single pixel
          at level N-1