> Hm, okay. I agree that this does give me the same semantics.
> But doesn't this degrade expected lookup performance to an O(n) list
> lookup if I put a lot of stuff into the hash which is (=) but not (==)?

Yes, of course.  It depends on your application.  For things like
hash-consing, this approach (standard hash function + comparison finer
than (=)) can still work well.  Alternatively, you can always put
unique integers in the data type used as key.

- Xavier Leroy