Query predefined prefixes and prefixes defined with
rdf_register_prefix/2 and local prefixes defined with
rdf_prefix/2. If Alias is unbound and one URI is the prefix of
another, the longest is returned first. This allows turning a
resource into a prefix/local couple using the simple enumeration
below. See rdf_global_id/2.
rdf_current_prefix(Prefix, Expansion),
atom_concat(Expansion, Local, URI),