if i >= 2 {
			w.Add(i - 1)
			w.Add(i - 2)
		}
		w.Add(i >> 1)
		w.Add((i >> 1) ^ 1)
	})
	if n != N+1 {
		t.Fatalf("ran %d items, expected %d", n, N+1)
	}
}

func TestWorkParallel(t *testing.T) {
	for tries := 0; tries < 10; tries++ {
		var w Work[int]
		const N = 100
		for i := 0; i < N; i++ {
			w.Add(i)
		}
		start := time.Now()
		var n int32
		w.Do(N, func(x int) {