Re: printing "a 2" at the beginning of staves automatically

[Werner LEMBERG]

>> Meanwhile, how about using a spanner rather than an item?  See
>> attached.

Aaand some nits :-)

* Contrary to Valentin's solution, the first 'a 2' text is not
  left-aligned to the left edge of the note head.  This is especially
  visible with whole notes (see attached image).  It is not clear to
  me why this is so, given that text is left-aligned if I do

  ```
  \override TextSpanner.bound-details.left.text = "rit."
  c’1\startTextSpan
  ```

  for example.

* `\tweak` doesn't work right before `\set Staff.staffAnnotation`.
  Instead, I have to use `\once \override`, AFAICS.

  Hopefully, I'm missing something.  Otherwise both Valentin's and
  Jeans' solutions are heavily limited because it happens quite often
  that the 'a 2' string must be positioned individually without
  affecting other rehearsal marks – or text spanners.


    Werner