Trouble coloring Ambitus ledger lines

[Patrick Karl]

On 9 Jul 2016 I submitted a report titled:  Problem coloring Ambitus ledger-lines.  A work-around quickly came (thanks Thomas Morley) consisting of inserting the following code at the beginning of the music:

  \once \override Staff.LedgerLineSpanner.color = #grey

  \grace s128

  \stopStaff \startStaff

I pointed out an immediate problem with that code that was given a further work-around (thanks Simon Albrecht) involving specifying the instrumentName in a \with block instead of using a \set command.

The following code:

\version "2.19.53"

S = {
  \once \override Staff.LedgerLineSpanner.color = #grey
  \grace s128
  \stopStaff \startStaff
  R1*3                   % 1-3
  \grace c'128
  c'1 c'''                  % 4
}

\score {
    \new Staff  { \compressMMRests \S }
    \layout {   \context { \Staff \consists "Ambitus_engraver" } }
}

shows two more problems with the suggested work-around, namely:

  *  as written the initial MM rests are not compressed , i.e., the \compressMMRests command has no effect.

  *  if the line containing the three rests is removed, then the compilation fails with the following assertion failure:

Interpreting music.../home/gub/NewGub/gub/target/darwin-x86/src/lilypond-git.sv.gnu.org--lilypond.git-release-unstable/lily/sequential-iterator.cc:163: failed assertion `!grace_fixups_ || grace_fixups_->start_ >= here_mom_'

and no output is obtained.

Hopefully, someone will have work-arounds for these two problems.

I would also be interesting in knowing why the Ambitus ledger lines are handled is such a different manner than, say, the AmbitusNoteHead, which doesn't give rise to any of these problems.  The amount of time I have spent dealing with problems arising from the original work-around must number in the dozens of hours.