class lookup_system_id :This is the generic builder for URL-based catalog resolvers: The catalog argument specifies pairs(string * resolver) list ->resolver
(url, r) mapping URL's identifiers to
subresolvers.
The subresolver is invoked if an entity with the corresponding URL
id is to be opened.
Important note: Two URL's are considered as equal if they are equal in their string representation. (This may not what you want and may cause trouble... However, I currently do not know how to implement a "semantic" comparison logic.)
Note: SYSTEM IDs are simply compared literally, without making
relative IDs absolute. See norm_system_id below for improving this.
