> From: Sora Takai <booksipadtest@gmail.com>
> Date: Mon, 19 Feb 2024 11:10:42 +0900
>
> 1. Are negative 'priority integers interpreted in an intuitive sense?  That is, would overlays with negative
> 'priority numbers rank distinctly lower than anything with higher 'priority values (0 or above)?
>
> 2. Are they "valid" in a sense that emacs intends 'priority property to be (in which case the manual description
> would have to be changed).

Negative priority values are valid, but used internally by Emacs.  So
we don't recommend using them in Lisp programs, as doing so might get
in the way of some Emacs features.

> Personally, I think having negative 'priority -- given it works as intended -- is useful, since I sometimes bump
> into use cases where I'd like to make sure certain overlays have the lowest 'priority possible below all others.

We recommend you not to use these values.