- rdf_db_to_file(+DB, -File) is det
- rdf_db_to_file(-DB, +File) is det
- Translate between database encoding (often an file or URL) and
the name we store in the directory. We keep a cache for two
reasons. Speed, but much more important is that the mapping of
raw --> encoded provided by www_form_encode/2 is not guaranteed
to be unique by the W3C standards.