Return a generator that yields the nodes of the tree in preorder.
        If skip_seed=True then the root node is not included.

def preorder(self, skip_seed=False):
        """
        Return a generator that yields the nodes of the tree in preorder.
        If skip_seed=True then the root node is not included.
        """
        for node in self._tree.preorder_node_iter():
            if skip_seed and node is self._tree.seed_node:
                continue
            yield node