Checkpoints (was re: Adding durations (for \after))

[Joel C. Salomon]

On 11/12/22 09:42, Kieren MacMillan wrote:
> Here's one concrete example: 
> https://lists.gnu.org/archive/html/lilypond-user/2010-12/msg00790.html
>
> “Annoyance: the expressive part needs skips of the durations corresponding to 
> the notes in between the expressive marks. Someday, I hope that LilyPond will 
> have the facility to mark temporal points in various parts so that they can be 
> aligned without counting out the beats correctly in each one.”

Is it even possible to query the temporal position, to enable some form 
of checkpoints?

Basic notion I'm thinking of is, \checkpoint <id> can appear in multiple 
staves, voices, etc., and gives a warning if it's not at the same 
temporal position across all of them.  Example:

\score {
  <<
    { a1 \checkpoint #17 b c \checkpoint #42 d }
    \\
    { d \checkpoint #17 e \checkpoint #42 f g }
  >>
}

`\checkpoint #17` succeeds; `\checkpoint #42` warns that it's in 
different positions across Voices.

(Motivation: I got bit by the "programming error: could not find this 
grob's vertical axis group in the vertical alignment" issue in one 
score.  Searching the list indicates this is probably a misalignment 
somewhere, but I've still not succeeded in pinning down a minimal 
example.  Cut the score in half, and both halves work anywhere I've 
thought to make the split.)

--Joel