Look up function:
class lookup_public_id_as_string :Makes a resolver for?fixenc:Pxp_types.encoding -> (string * string) list ->resolver
PUBLIC identifiers. The catalog argument specifies
pairs (pubid, text) mapping PUBLIC identifiers to XML text (which must
begin with <?xml ...?>).
The same restriction as for lookup_public_id_as_file applies.
fixenc: Overrides the encoding of the strings.
