* 'SHORTEN', full uris are shortened using prefixes for property names, relationship names and labels. Prefixes can be defined upfront and for those undefined, n10s will dynamically generate one of the form `ns1`,`ns2`...
* 'SHORTEN_STRICT', same behavior `'SHORTEN'` but fails if a prefix is not predefined for a namespace in the imported RDF.
* 'IGNORE' uris are ignored and only local names are kept
* 'MAP' vocabulary element mappings are applied on import
* 'KEEP' uris are kept unchanged