Pre-order search of the tree rooted at this node.
        (First visit current node, then visit children.)

        :rtype: generator of :class:`~aeneas.tree.Tree`

def pre(self):
        """
        Pre-order search of the tree rooted at this node.
        (First visit current node, then visit children.)

        :rtype: generator of :class:`~aeneas.tree.Tree`
        """
        yield self
        for node in self.children:
            for v in node.pre:
                yield v