for y := 0; y <= size-len; y++ {
				for diffpos := x; diffpos < x+len; diffpos++ {
					a[diffpos] = 1
					if Equal(a[x:x+len], b[y:y+len]) || Equal(b[y:y+len], a[x:x+len]) {
						t.Errorf("NotEqual(%d, %d, %d, %d) = true", len, x, y, diffpos)
					}
					a[diffpos] = 0
				}
			}
		}
	}
}

var indexTests = []BinOpTest{
	{"", "", 0},
	{"", "a", -1},
	{"", "foo", -1},