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It was thus said that the Great Tony Papadimitriou once stated:
> Hi all,
> I’m looking for the best (?) general way to implement *nested* iterators
> in Lua (5.3, of course).
> For example:  I have an iterator that returns single items when used in a
> FOR .. IN loop.  Each item is treated somehow but that loop, and the
> result I want it to become available in another iterator.
> Easy, but I do NOT want to store all intermediate results in a table, and
> then traverse the table again, as the size of the table could be too large
> or even unknown while the iterator is running (until some condition
> happens that terminates it).
> In Python 3, for example, this would be very simple with each routine
> using YIELD instead of RETURN, and iterators can be nested to an arbitrary
> level.  But, in Lua, I can’t seem to find an obvious (memory efficient)
> solution.  (I would love for Lua to have the same simplicity in this
> regard.)
> Perhaps explicit use of co-routines is one solution, but maybe there is a
> more immediate way I missed.
> Maybe someone can point me to some example.

  Here's an example that I think answers what you are asking:

	function iterator(a,b)
	  return coroutine.wrap(function()
	    for ai = 1 , #a do
	      for bi = 1 , #b do	

	A = { 1 , 2 , 3 , 4 , 5 }
	B = { "one" , "two" , "three" , "four" , "five" }
	for item in iterator(A,B) do