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The following self-referencing generator has incorrect output:
def ab_combinations():
 #'', 'a', 'b', 'aa', 'ab', 'ba', 'bb', 'aaa', ...
 def _deferred_output():
 yield ""
 tees = tee(output)
 #This definition works fine: '', 'a', 'b', 'aa', 'ab', 'ba', ...
 l = [(item+"a" for item in tees[0]), (item+"b" for item in tees[1])]
 #This definition results in: '', 'b', 'b', 'bb', 'bb', 'bb', ...
 #l = [(item+label for item in t) for t, label in zip(tees,"ab")]
 
 while True:
 for g in l:
 yield next(g)
 result, output = tee(_deferred_output())
 return result