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> There is one other issue: What generator would you recommend
> for a Scheme system with only 28-bit (or something) immediate
> integers (and boxed arbitrary integers) and no floating point number

Presumably this is way to complex/slow, but there is a PRNG called Yarrow
described in http://www.counterpane.com/yarrow-notes.html.
It uses SHA1 and DES to generate bits.

bengt