[plt-scheme] partial orders and preorders

Sorry, I don't...  But unless you quickly find an all-around better
implementation, please feel encouraged to put up on the Web whatever
you've written, even though you suspect it's not the most efficient.

(As a recent pleasant side-project that can be done on the laptop from
park benches, I'm trying to pound out small reusable R5RS
implementations of various algorithms, without worrying too much about
whether I have the end-all-be-all algorithm -- just to get *an*
algorithm out there where people who've used the algorithm in Perl,
Python, CL, etc., can Google for it in Scheme.  For example, I coded up
Levenshtein Distance, even though I understand there exist more exotic
edit-distance algorithms that are better for at least some classes of
input.  I or someone else will get to the fancier algorithms as needed,
but an LD algorithm right now seemed useful.  I'll try not to be too
embarrassed when someone inevitably says, "I don't know why Neil
bothered to code up LevD when the lesser-known FooD is superior in all
respects.  If only he'd spent a day at the library, scouring the
literature!  Here is a FooD implementation...")