code: def _all_expander(fringe, iteration, viewer):
      '''
      Expander that expands all nodes on the fringe.
      '''
      expanded_neighbors = [node.expand(local_search=True)
                            for node in fringe]
  
      if viewer:
          viewer.event('expanded', list(fringe), expanded_neighbors)
  
      list(map(fringe.extend, expanded_neighbors)f _all_expander(fringe, iteration, viewer):
      '''
      Expander that expands all nodes on the fringe.
      '''
      expanded_neighbors = [node.expand(local_search=True)
                            for node in fringe]
  
      if viewer:
          viewer.event('expanded', list(fringe), expanded_neighbors)
  
      list(map(fringe.extend, expanded_neighbors))
docs: Expander that expands all nodes on the fringe.