From the chapter, accumulation of a recursive data structure by use of a Queue.

Summing with a Queue

Q = [[2,1,9],3,[[6,7],0]]
def loopsum(List):
  Accum, Queue = 0, List[:]
  # Queue starts out with all items
  while len(Queue)>0:
    if type(head)==int:
      Accum += head
    else: # head is a list
      # move nested items to end
  # Magically, when loop ends, done!
  return Accum
print loopsum(Q)