Return the (relation name, parent ID) tuple that a node is in.
        Return None if this node is not in a relation.

def get_relname_and_parent(self, treepos):
        """Return the (relation name, parent ID) tuple that a node is in.
        Return None if this node is not in a relation.
        """
        node = self.dgtree[treepos]
        node_type = get_node_type(node)
        assert node_type in (TreeNodeTypes.relation_node, TreeNodeTypes.leaf_node)

        parent_pos = self.get_parent_treepos(treepos)
        if parent_pos is None:  # a root node has no upward relation
            return None, None
        else:
            parent_label = self.get_parent_label(treepos)
            grandparent_pos = self.get_parent_treepos(parent_pos)

            if grandparent_pos is None:
                # a tree with only one EDU/leaf and a 'N' parent but no relation
                return None, None
            else:
                grandparent_id = self.get_node_id(grandparent_pos)
                grandparent_label = self.get_parent_label(parent_pos)
                reltype = self.get_reltype(grandparent_label)

                if reltype == 'rst':
                    if parent_label == 'N':
                        return 'span', grandparent_id
                    elif parent_label == 'S':
                        cousins_pos = self.get_cousins_treepos(treepos)
                        assert len(cousins_pos) == 1
                        cousin_id = self.get_node_id(cousins_pos[0])
                        return grandparent_label, cousin_id
                elif reltype == 'multinuc':
                    return grandparent_label, grandparent_id