class lookup_system_id_as_file :Looks up resolvers for URL identifiers: The catalog argument specifies pairs
?fixenc:Pxp_core_types.I.encoding -> (string * string) list ->
(url, filename)mapping URL's to filenames. The filenames must already be encoded in the character set the system uses for filenames.
Note: URL's are simply compared literally, without making
relative IDs absolute. See
norm_system_id below for improving this.
fixenc: Overrides the encoding of the file contents. By default, the
standard rule is applied to find out the encoding of the file.