bool next_node(tree *&to_return);
//!< sets "to_return" to the next tree in the direction of tree traversal.
/*!< if the next node could not be found in one invocation of next_node,
bool next_node(tree *&to_return);
//!< sets "to_return" to the next tree in the direction of tree traversal.
/*!< if the next node could not be found in one invocation of next_node,