@Override
    public double variance() {
        return PI2_3 * scale * scale;
    }
@Test
    public void testVariance() {
        System.out.println("variance");
        LogisticDistribution instance = new LogisticDistribution(2.0, 1.0);
        instance.rand();
        assertEquals(Math.PI*Math.PI/3, instance.variance(), 1E-7);
    }