def METHOD_NAME(self, tree_size=None):
    """Returns the root hash of the tree denoted by |tree_size|."""
    if tree_size is None:
        tree_size = self.tree_size()
    if tree_size > self.tree_size():
        raise ValueError("Specified size beyond known tree: %d" % tree_size)
    return self.__hasher.hash_full_tree(self.__leaves[:tree_size])