def _branch_descendants(self, branch=None):
        """Iterate over all branches immediately descended from the current
        one (or the given one, if available).

        """
        branch = branch or self.branch
        for (parent, (child, _, _, _, _)) in self._branches.items():
            if parent == branch:
                yield child
Iterate over all branches immediately descended from the current
        one (or the given one, if available).