PublicShow -- Yadis discovery

See also
Source xrds_dom(+Id, -XRDS_DOM) is det
True when XRDS_DOM is a parsed XML document for the given resource.
Source xrds_location(+Id, -XRDSLocation) is semidet
Discover the location of the XRDS document from the given Id.
Source xrds_specified_location(+URL, -XRDSLocation) is nondet[multifile]
Hook that allows for specifying locations of XRDS documents. For example, Google does not reply to Yadis discovery messages. We can fake it does using:

If this hook succeeds with XRDSLocation bound to - (minus), we assume there is no XRDS document associated to URL. This can be used to avoid retrieving misleading or broken XRDS documents.