@Override
    public double sd() {
        return sigma;
    }
@Test
    public void testSd() {
        System.out.println("sd");
        GaussianDistribution instance = new GaussianDistribution(0.0, 1.0);
        instance.rand();
        assertEquals(1.0, instance.sd(), 1E-7);
        instance = new GaussianDistribution(1.0, 2.0);
        instance.rand();
        assertEquals(2.0, instance.sd(), 1E-7);
        instance = new GaussianDistribution(2.0, 0.5);
        instance.rand();
        assertEquals(0.5, instance.sd(), 1E-7);
        instance = new GaussianDistribution(3.0, 3.8);
        instance.rand();
        assertEquals(3.8, instance.sd(), 1E-7);
    }