Shortcut for \repeat unfold

[Jean Abou Samra]

Hi,

This continues a discussion sparked at
https://gitlab.com/lilypond/lilypond/-/merge_requests/921

In short, I propose to make the first argument to
\repeat optional, making \repeat n music equivalent to
\repeat unfold n music.

The principal motivation is sparing typing. \repeat unfold
is commonly used as a time-saving shortcut, not only in
real-world usage, but also while constructing minimal
working examples involving filler music. It is personally
one of the commands I use the most frequently, and I am
apparently not alone in that case
(https://gitlab.com/lilypond/lilypond/-/merge_requests/921#note_682103342).
My experience is that every time I want to repeat a note
four times, I have a half-a-second wonder about whether
typing "\repeat unfold" will be shorter than cut-and-paste.

Now, if we want to shorten \repeat unfold, what's the best
syntax? I favor \repeat as it matches usage patterns of
functions with optional arguments (like \time). Even though
\repeat is not currently a music function, I think having it
mimic that syntax helps mnemonics. \repeat n music is also
very readable.

Other possibilities:

- "\rep n music", not as self-telling (and I think Lukas
  only calls his shortcut \rep because redefining \repeat
  is not syntactically valid);
- "music * n", attractive but the difference between c1*5
  and { c1 } * 5 would be confusing; also, for longer passages
  one would rather state the number of repetitions from the
  start than having to remember to put it at the end;
- "\* n music", very short but doesn't read as nicely I think,
  and I don't view repeats as special enough to warrant
  this kind of syntax with many special characters.

With this change, documentation can be updated to use
the short syntax (let's leave Documentation/snippets as-is
though, to avoid piling work that will have to be paid
at the next LSR upgrade, whenever that happens).

Thoughts?

Regards,
Jean

PS: Apart from regression tests and documentation,
I have a draft implementation locally.