ISA function?

Is there any PostgreSQL function, that, given an OID and a table (class)
name, would true true (or the OID) if the OID is a member of that class?

I want to write something like:

create function ISA(oid, name) returns oid as '
select oid from $2* where oid = $1
' language 'sql';

(But, of course, this doesn't work).

--
Will Fitzgerald
I/NET, Inc.