(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.)
SYSTEM IDs are simply compared literally, without making
relative IDs absolute. See
norm_system_id below for improving this.