docs: def _set_unique_id(self, unique_id):
          """Define a unique id"""
          assert isinstance(unique_id, int), "unique_id must be an integer"
  
          if PolygonFilter.instace_exists(unique_id):
              newid = max(PolygonFilter._instance_counter, unique_id+1)
              msg = "PolygonFilter with unique_id '{}' exists.".format(unique_id)
              msg += " Using new unique id '{}'.".format(newid)
              warnings.warn(msg, FilterIdExistsWarning)
              unique_id = newid
  
          ic = max(PolygonFilter._instance_counter, unique_id+1)
          PolygonFilter._instance_counter = ic
          self.unique_id = unique_id