class lookup_id_as_file :The list (catalog) argument specifies pairs
?fixenc:Pxp_core_types.I.encoding -> (Pxp_core_types.I.ext_id * string) list ->
(xid,file)mapping external IDs
xidto files. The file is read 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.
fixenc: Overrides the encoding of the file contents. By default, the
standard rule is applied to find out the encoding of the file.