docs: def get_siblings_treepos(self, treepos):
          """Given a treeposition, return the treepositions of its siblings."""
          parent_pos = self.get_parent_treepos(treepos)
          siblings_treepos = []
  
          if parent_pos is not None:
              for child_treepos in self.get_children_treepos(parent_pos):
                  if child_treepos != treepos:
                      siblings_treepos.append(child_treepos)
          return siblings_treepos