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Here is a first cut at a patch. There is one slight deviation from the original spec:
> some nice error checking for when the generator's frame is already gone > (or the supplied object isn't a generator iterator).
The attached patch returns empty mappings for these cases. I can easily
add the error checks, but in what cases is it useful to know *exactly*
why a mapping could not be created? Having an empty mapping for all 
invalid cases is simpler and seems more robust.