warning: programming error: must have Item for spanner bound of PercentRepeat

[Daniel Hulme]

I'm trying to re-engrave a percussion part I have been given to play and
I get the above message, with a superfluous bar in my output. A minimal
example and Lilypond's resulting message follows. I started with 2.10.33
but updated to git master on Thursday night and the problem persists.
Even if the problem is with my file it seems that reporting an alleged
programming error must be helpful. I can try to fix the bug myself if
someone points me in the right direction.

Incidentally, it would be nice if there were some way to set the value
of the counter on percent repeats, so (for instance) if the first bar of
a phrase is slightly different from the second and subsequent bars, the
first percent mark is numbered 3 to agree with the phrase structure
(whereas now it would be 2). Also, a feature to only number every n'th
percent mark would also be nice. Both of these properties are found in
commercial parts, though less often than I would like.

=== percentrepeat.ly
\version "2.11.62"
\include "english.ly"

basicSnare = \drummode { sn8 sn sn-> sn }
basicBass = \drummode { bd8[ r16 bd16] bd8 r8 }

basicBar = \drummode {
  \repeat unfold 2 {
    << \basicSnare \\ \basicBass >>
  } |
}

piece = \drummode {
  \time 4/4
  \repeat percent 10 \basicBar
  % ...
}

skips = #(ly:export (skip-of-length piece ))

\score {
  \new DrumStaff <<
    \new DrumVoice = "1" { \skips }
    \new DrumVoice = "2" { \skips }
    \piece
  >>
}

=== output.log

GNU LilyPond 2.11.63
Processing `percentrepeat.ly'
Parsing...
Interpreting music... 
percentrepeat.ly:15:2: warning: programming error: must have Item for spanner 
bound of PercentRepeat
  
  \repeat percent 10 \basicBar
[8]
Preprocessing graphical objects...
programming error: Multi_measure_rest::get_rods (): I am not spanned!
continuing, cross fingers
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `percentrepeat.ps'...
Converting to `./percentrepeat.pdf'...

=== end

-- 
“After all, one can't complain.  I have my friends. Somebody spoke to me
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bumped into  me and said  ‘Bother!’  The Social Round.  Always something
going on.”  -- A. A. Milne, ‘Winnie-the-Pooh’  http://surreal.istic.org/

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