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Oh ho. Am I right in fearing that the idea of precomputing the iterables was broken in the first place? We just cannot do this. The things we are looping over may depend on other stuff from the generator expression itself. Example:

>>> [x for l in [[1,2],[3,4]] for x in l]
[1, 2, 3, 4]
>>> (y for m in [[1,2],[3,4]] for y in m)
NameError: name 'm' is not defined