class lookup_id_as_string :The list (catalog) argument specifies pairs?fixenc:Pxp_types.encoding -> (Pxp_types.ext_id * string) list ->resolver
(xid,s) mapping external IDs xid
 to strings s. The string is read 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.