code: def demote(self, move: chess.Move) -> None:
          
          variation = self[move]
          i = self.variations.index(variation)
          if i < len(self.variations) - 1:
              self.variations[i + 1], self.variations[i] = self.variations[i], self.variations[i + 1]
docs: Moves a variation one down in the list of variations.