code: def backup_value(self, value, up_to):
          
          self.N += 1
          self.W += value
          if self.parent is None or self is up_to:
              return
          self.parent.backup_value(value, up_to)
docs: Propagates a value estimation up to the root node.
  
          Args:
              value: the value to be propagated (1 = black wins, -1 = white wins)
              up_to: the node to propagate until.