t.Errorf("at iteration %d got %d, %d want %d, %d", cnt, i, v, ei, ev)
			}
			ei++
			ev++
			cnt++
		}
		if cnt != size {
			t.Errorf("read %d values expected %d", cnt, size)
		}
	}
}

func TestBackward(t *testing.T) {
	for size := 0; size < 10; size++ {
		var s []int
		for i := range size {
			s = append(s, i)
		}
		ei, ev := size-1, size-1
		cnt := 0
		for i, v := range Backward(s) {