'Step forward in the tree, keeping everything we already know about the subtree, assuming
that get_move() has been called already. Siblings of the new root will be garbage-collected.'

def update_with_move(self, last_move):

    if (last_move in self._root._children):
        self._root = self._root._children[last_move]
        self._root._parent = None
    else:
        self._root = TreeNode(None, 1.0)
