docs: def xdifference(self, to):
          """ Generator
              Gives the difference of quadkeys between self and to
              Generator in case done on a low level
              Only works with quadkeys of same level
          """
          x,y = 0,1
          assert self.level == to.level
          self_tile = list(self.to_tile()[0])
          to_tile = list(to.to_tile()[0])
          if self_tile[x] >= to_tile[x] and self_tile[y] <= self_tile[y]:
              ne_tile, sw_tile = self_tile, to_tile
          else:
              sw_tile, ne_tile = self_tile, to_tile
          cur = ne_tile[:]
          while cur[x] >= sw_tile[x]:
              while cur[y] <= sw_tile[y]:
                  yield from_tile(tuple(cur), self.level)
                  cur[y] += 1
              cur[x] -= 1
              cur[y] = ne_tile[y]