class lookup_id :The general catalog class. The list (catalog) argument specifies pairs(Pxp_types.ext_id * resolver) list ->
resolver
(xid,r)
mapping external IDs xid
to subresolvers r
. The subresolver is invoked
if an entity with the corresponding xid
is to be opened.
Note: SYSTEM
IDs are simply compared literally by this class,
without making
relative IDs absolute. See norm_system_id
below for how to improve this.