code: def add_virtual_loss(self, up_to):
          
          self.losses_applied += 1
          # This is a "win" for the current node; hence a loss for its parent node
          # who will be deciding whether to investigate this node again.
          loss = self.position.to_play
          self.W += loss
          if self.parent is None or self is up_to:
              return
          self.parent.add_virtual_loss(up_to)
docs: Propagate a virtual loss up to the root node.
  
          Args:
              up_to: The node to propagate until. (Keep track of this! You'll
                  need it to reverse the virtual loss later.)