code: def leaves(self, nodes=None, unique=True):
          
          if nodes is None:
              return super(DependencyTree, self).leaves(unique=unique)
  
          res = list()
          for child_id in nodes:
              for sub_child in self._all_nodes[child_id].leaves(unique=unique):
                  if not unique or sub_child not in res:
                      res.append(sub_child)
          return res
docs: Get the leaves of the tree starting at this root.
  
          Args:
              nodes (iterable): limit leaves for these node names
              unique: only include individual leaf nodes once
  
          Returns:
              list of leaf nodes