# Class Pxp_reader.norm_system_id

    class norm_system_id : resolver -> resolver

Normalizes URL's, and forwards the open request to the passed resolver. (Non-URL ID's are forwarded unchanged to the subresolver.)

Normalization includes:

- Relative URLs are made absolute. If this fails, the problematic relative URL will be rejected.
- `..` and `.` and `//` in the middle of URLs are removed
- Escaping of reserved characters is normalized (percent encoding like %40)

Normalization is recommended for catalogs, e.g. `new norm_system_id
(new lookup_system_id_as_file
[ "http://h/p1", ...;
"http://h/p2", ...;
])
` First, the catalog now even works if the URL is written in an unsual way, e.g. `http://h/p1/../p2`, or `http://h/p%31`. Second, relative URLs can be used. For instance, the document referred to as `http://h/p1` can now refer to the other document as `p2`.