@Test public void prefixes1() {

    // All prefixes used.
    String data1 = StrUtils.strjoinNL
        ("@prefix : <http://example/> ." ,
         "@prefix ex: <http://example/ex#> ." ,
         "" ,
         ":s1 :p :x1 ." ,
         ":s1 ex:p :x1 ."
         ) ;
    Graph graph1 = create(data1) ;
    PrefixMapping pmap = PrefixMappingUtils.calcInUsePrefixMapping(graph1) ;
    PrefixMapping pmapExpected = graph1.getPrefixMapping() ;
    Assert.assertEquals(2, size(pmap)) ;
    Assert.assertEquals(pmapExpected, pmap) ; 
}