docs: def get_children_treepos(self, treepos):
          """Given a treeposition, return the treepositions of its children."""
          children_treepos = []
          for i, child in enumerate(self.dgtree[treepos]):
              if isinstance(child, nltk.Tree):
                  children_treepos.append(child.treeposition())
              elif is_leaf(child):
                  # we can't call .treeposition() on a leaf node
                  treepos_list = list(treepos)
                  treepos_list.append(i)
                  leaf_treepos = tuple(treepos_list)
                  children_treepos.append(leaf_treepos)
          return children_treepos