[plt-scheme] using syntax objects as hash table keys

I'd like to use syntax objects as keys in a hash table.  The problem
is, I have no idea how to compute a hash code such that
`module-identifier=?' identifiers map to the same code.  Anyone have
any ideas?  I can't even use the symbol (i.e. `(syntax-e id)') as the
key and have an alist for each symbol, since identifiers with
different symbols might be `module-identifier=?' due to renaming.
(Hm, can I just use `(cadr (identifier-binding id))' in that case?)

--Doug