public Stopwatch stop() {
        if (isRunning()) stopTime = Instant.now();
        return this;
    }
@Test
    public void addSubstract() {
        Instant i1 = Instant.now();
        Instant i2 = i1.plus(stopwatch.stop());
        Instant i3 = i2.minus(stopwatch);
        assertEquals(i1, i3);
        assertTrue(i2.compareTo(i1) >= 0);
        assertTrue(i3.compareTo(i2) <= 0);
    }