Extracted from #19

It's probably linked to signed/unsigned conversion, but trying to isolate the bug removes it. It might require int conversion via Generics in a file that doesn't directly import sparsesets or something like that.

Symptoms:

Note that even with asserts off we still get issues. Some thief/victim ID or sparset length appear negative if converted to "int" or normal (in the 250 range) if kept as "Setuint".

I don't now why it doesn't happen on master.