code: def addend_idxs(self):
          
          return tuple(
              self.valid_elements.get_by_id(addend_id).index_in_valids
              for addend_id in self.addend_ids
          )
docs: tuple of int index of each addend element for this subtotal.
  
          The length of the tuple is the same as that for `.addend_ids`, but
          each value repesents the offset of that element within the dimension,
          rather than its element id.