public Frequency subtract(Frequency value) {
        return new Frequency(this.frequency - value.frequency);
    }
@Test
    public void testSubtract() {
        Frequency high = Frequency.ofGHz(1);
        Frequency low = Frequency.ofMHz(100);
        Frequency expected = Frequency.ofMHz(900);

        assertThat(high.subtract(low), is(expected));
    }