@Test
 public void test_massInG() {
   m1 = Values.massInKg(1);
   m2 = Values.massInG(1000);
   m3 = Values.massInG(500);
   m4 = Values.massInG(2500);
   m5 = Values.massInG(1500);
   m6 = Values.massInG(2000);
   assertEquals(m4, m3.mul(5));
   assertEquals(m3, m4.div(5));
   assertEquals(m5, m1.add(m3));
   assertEquals(m6, m4.sub(m3));
   assertTrue(!m6.isZero());
   assertTrue("0,50".equals(m3.toString()));
   assertEquals(m1, m2);
 }