docs: def get_children_labels(self, treepos):
          """Given the treeposition of a node, return the labels of its children."""
          children_labels = []
          node = self.dgtree[treepos]
          for child in node:
              if is_leaf(child):
                  # we can't call .label() on a leaf node
                  children_labels.append(child)
              else:
                  children_labels.append(child.label())
          return children_labels