Hm, this gets a little funny when `by` is used.

```coffee
indexes = (index for n, index in [5..-5] by -5)
```

What should `indexes` be here? If you imagine that the range is actually an array, then it should be `[0, 5, 10]`, though it's pretty weird that the indexes go up even though the step is negative.

If you instead think of the range as an actual range on a number line, as opposed to an array, then the whole concept of an index is kinda wacky. Maybe that's what @jashkenas was thinking when he disallowed this construct?