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