Matt Armstrong <matt@rfc20.org> writes:

> I noticed there was no explicit test against the overlay modification
> hook boundary conditions.  The attached patch adds some, which might
> be nice to have if a person (possibly me) someday optimizes overlay
> performance with a change in data structure.
>
> In writing this I was surprised to learn that the hooks do not behave
> differently if the overlay is `front-advance' or `rear-advance', so
> this is explicitly tested.

...follow up with a comment typo fix.