> 1) I don't have any strong feeling about using one or two values. But in
> any case they should be configurable. This will add more use cases that
> could benefit with this and I think it will be simple to implement (just
> need to add 2 int variables).

Adding the variables is trivial, yes. The problem is dealing with errors, like the user setting the variables to non-numbers, etc. I'm sure we don't want to throw signals there, do we?

Eli, opinions?

> 2) Is it possible to call merge_faces more lazily? I mean inside the
> `if` `else if` so only one of them will be called when needed. AFAIK
> maybe_produce_line_number is called for every line and the code needs
> only one of those merges every 5/10/x lines.

You're right. Thanks. Improved patch attached.