@Test
public void testExec() {

    NodeValue v1 = NodeValue.makeNode("<http://www.opengis.net/def/crs/EPSG/0/27700> POLYGON((140 15, 140 45, 200 45, 200 15, 140 15))", WKTDatatype.INSTANCE);
    NodeValue v2 = NodeValue.makeNode("<http://www.opengis.net/def/crs/EPSG/0/27700> POLYGON((145 30, 145 40, 160 40, 160 30, 145 30))", WKTDatatype.INSTANCE);
    DifferenceFF instance = new DifferenceFF();
    NodeValue expResult = NodeValue.makeNode("<http://www.opengis.net/def/crs/EPSG/0/27700> POLYGON((140 15, 140 45, 200 45, 200 15, 140 15), (145 30, 160 30, 160 40, 145 40, 145 30))", WKTDatatype.INSTANCE);
    NodeValue result = instance.exec(v1, v2);
    assertEquals(expResult, result);
}