# Class Pxp_reader.lookup_id

    class lookup_id : (Pxp_types.ext_id * resolver) list -> resolver

The general catalog class. The list (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.

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.