code: def _bin_update_items(self, items, replace_at_most_one,
                            replacements, leftovers):
          
          for key, value in items:
              # If there are existing items with key <key> that have yet to be
              # marked for replacement, mark that item's value to be replaced by
              # <value> by appending it to <replacements>.
              if key in self and key not in replacements:
                  replacements[key] = [value]
              elif (key in self and not replace_at_most_one and
                    len(replacements[key]) < len(self.values(key))):
                  replacements[key].append(value)
              else:
                  if replace_at_most_one:
                      replacements[key] = [value]
                  else:
                      leftovers.append((key, value))
docs: <replacements and <leftovers> are modified directly, ala pass by
          reference.