So I started writing a couple different options and wasn't happy with either of them. I really don't like the idea of falling back on anything.

- With the existing changes, the segment options are simply "Specify a URI segment, or we'll use the last one as a default". Adding more fallbacks just complicates the docs, and potentially makes it harder for devs to track the logic.
- segment(3) or rsegment(3) won't necessarily have the pagination param, even if it returns a valid number (e.g. *ci.dev/blog/2013/01/28/2*). segment(3) we already knew sucked to begin with as a default, and rsegment() needs to be setup properly to be consistent. Similar to first point, that's just more the developer probably doesn't need to know or do.
- Falling back means doing twice the sanity checks. With what's there now, we can just replace `is_numeric()` with `ctype_digit()` and pretty much be done with it.