# Class Pxp_reader.lookup_system_id

    class lookup_system_id : (string * resolver) list -> resolver

This is the generic builder for URL-based catalog resolvers: The catalog argument specifies pairs `(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.