class lookup_system_id_as_string :Looks up resolvers for URL identifiers: The catalog argument specifies pairs?fixenc:Pxp_types.encoding -> (string * string) list ->resolver
(url, text) mapping URL's to XML text (which must
begin with <?xml ...?>).
Note: URL's are simply compared literally, without making
relative IDs absolute. See norm_system_id below for how to improve this.
fixenc: Overrides the encoding of the strings.
