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.
