class lookup_id :The general catalog class. The list (catalog) argument specifies pairs
(Pxp_types.ext_id * resolver) list ->
(xid,r)mapping external IDs
r. The subresolver is invoked if an entity with the corresponding
xidis to be opened.
SYSTEM IDs are simply compared literally by this class,
relative IDs absolute. See
norm_system_id below for how to improve this.