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.
