docstr: Returns a generator which visits all nodes in expression tree.
  
  Args:
      bfs (bool): if True, use breadth-first search (BFS) traversal order;
          if False, use the depth-first search (DFS) traversal order
  
  Returns:
      the generator object.
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code: def walk(self, bfs=True):
      """
      Returns a generator which visits all nodes in expression tree.
  
      Args:
          bfs (bool): if True, use breadth-first search (BFS) traversal order;
              if False, use the depth-first search (DFS) traversal order
  
      Returns:
          the generator object.
      """
      if bfs:
          yield from self.bfs()
      else:
          yield from self.dfs()
  
imports: from abc import ABC, abstractmethod
  from collections import deque
  from copy import deepcopy
  from enum import IntEnum, auto, unique
  from typing import Any
label_desc: Use image node_id 14 for calling the AbstractExpression obj's underlying member method code with example usage: obj.walk(bfs) without return types