Color lexer backup distances and `dont-stop`

[LiberalArtist]

@mflatt

I think the history is that the need for backup was noticed before the need for something like dont-stop. When dont-stop was added later, it wasn't immediately obvious that the two always need to go together. As far as I can tell now, they need to be used in tandem.

I'm persuaded that needing a backup distance always implies needing dont-stop, but I thought the following was an example of needing dont-stop not necessarily implying the need for a non-zero backup distance (I could be wrong, though!):

@LiberalArtist

If you only need to peek into the next token, as with [a-] vs. [a-z], IIUC you need dont-stop but don't need a backup distance.

(I'm assuming that in [a-] you want to color - as a literal character, but in [a-z] you want to color it as an infix operator.)

In the course of looking into this, I had also come across @greghendershott's call for an example of dont-stop in use:

Example of a lang lexer that returns a `dont-stop` struct?

A lang's module lexer can return a dont-stop struct. I would like to test my handling of that. But so far (with e.g. langs racket, rhombus, and scribble/manual) I'm never seeing a dont-stop. What's an example of such a #lang? What's some minimal example program in that #lang eliciting the dont-stop value?

Since no one came up with an example, it might be that there's more ability to make changes without causing problems in practice than I would otherwise assume.