def master():
  gens = [None, None]
  gens[0] = funA(gens)
  gens[1] = funB(gens)
  gen = gens[0] # start with A
  try:
    while True:
      gen = gen.next()
  except StopIteration:
    None

def funA(gens):
  print "In A, right before first resume B."
  yield gens[1]
  print "In A, right before second resume B."
  yield gens[1]
  print "In A, right before third resume B."
  yield gens[1]

def funB(gens):
  print "In B, right before first resume A."
  yield gens[0]
  print "In B, right before second resume A."
  yield gens[0]
  print "In B, right at the end."