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Boosted priority because this is an especially bad kind of 
bug:  mysterious, subtle, and very rare ("most" in-range 
floats pack under "<f" and ">f" without problems; a problem 
requires that a carry out of the 25th most-significant-bit 
propagate thru a solid string of 24 1 bits).

For 2.2 I expect to check in a quick hack.  In 2.3 this code 
needs refactoring (structmodule and cPickle shouldn't have 
duplicated this delicate code)