docs: def _check_single_outgroup(self):
          """
          If only one (or none) of the seed node children is not a leaf node
          it is not possible to prune that edge and make a topology-changing
          regraft.
          """
          root_child_nodes = self.tree._tree.seed_node.child_nodes()
          not_leaves = np.logical_not([n.is_leaf() for n in root_child_nodes])
          if not_leaves[not_leaves].size <= 1:
              return [root_child_nodes[np.where(not_leaves)[0]].edge]
          return []