@Test public void prefixes4() {

    // No prefixes.
    String data = StrUtils.strjoinNL
        (
         "<http://other/s1> <http://example/p> 123 ."
         ) ;
    Graph graph1 = create(data) ;
    PrefixMapping pmap = PrefixMappingUtils.calcInUsePrefixMapping(graph1) ;
    Assert.assertEquals(0, size(pmap)) ;
    PrefixMapping pmapExpected = new PrefixMappingImpl() ;
    Assert.assertTrue(sameMapping(pmapExpected, pmap)) ;
}