docs: def get_cousins_treepos(self, treepos):
          """Given a treeposition, return the treeposition of its siblings."""
          cousins_pos = []
  
          mother_pos = self.get_parent_treepos(treepos)
          if mother_pos is not None:
              aunts_pos = self.get_siblings_treepos(mother_pos)
              for aunt_pos in aunts_pos:
                  cousins_pos.extend( self.get_children_treepos(aunt_pos) )
          return cousins_pos