Easy way to find out allowed data?

Hi,

Is there any convenient way to get what input qualifies as valid data
for a CHECK constraint? I'm trying to generalise an interface which
allows data entry through the Web. I'd like to be able to find out
what data is allowed in a field, and then generate a pick-list based
on that. It looks like relevant information is in pg_relcheck, but
I'm not sure I understand what I'd have to do to get it out (more
like, "I'm sure I don't understand. . ."), because there's a lot of
other stuff in there.

Thanks,
A
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