[issue2183] optimize list comprehensions

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

This looks like a major improvement, not just for speed, but for getting
rid of the _[1] arguments setup, retrieval, and deletion.

I presume it has been tested with nested and conditional variants? 

     def f(): return [x for t in s for x in t if x]

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nosy: +rhettinger

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