Re: Differences in `ne` and `bp` line-breaking behavior

[onf]

On Wed Dec 4, 2024 at 8:46 PM CET, Peter Schaffter wrote:
> On Wed, Dec 04, 2024, Douglas McIlroy wrote:
> > Also .ne is effective in the absence of traps, a fact that groff(7)
> > misses, too.
>
> How about
>
> .ne d   Spring the next vertical postion trap if it is nearer than
>         distance d (default scaling unit v).  In the absence of a
>         trap, break to a new page if page bottom is nearer than d.
>
> Removes grammatical ambiguity and covers all the bases.

As others noted, groff creates an implicit trap at the position
corresponding to page length, which causes a page break.

>From groff(7):
  The implicit page trap
    An implicit page trap always exists in the top-level diversion; it
    works like a trap in some ways but not others.  Its purpose is to
    eject the current page and start the next one.  It has no name, so
    it cannot be moved or deleted with wh or ch requests.  You cannot
    hide it by placing another trap at its location, and can move it
    only by redefining the page length with .pl.  Its operation is
    suppressed when vertical page traps are disabled with the vpt
    request.

~ onf