Bug when using acciaccatura and polyphony?

[Ismael Cortes]

Hello, I've been writing a python script to transform drum tablatures 
into lilypond (and then to PDF). After implementing polyphony to the 
script and testing some tablatures I realized that in the log it showed 
the text:

"programming error: moving backwards in time
continuing, cross fingers"

(no quotes, of course)

and the resulting PDF started looking as if the (Auto_)Beam_engraver has 
been removed (ie no auto Beams where shown), but only after the fault 
measure. It just seems to happen when the voice starts with a 
\acciaccatura grace note. It doesn't matters if it's the first or the 
second voice and adding \voiceOne nor \voiceTwo after the grace note 
doesn't helps.

I append a partial file transformed by my script which shows the error 
(I'm not sure if I can legally append the whole file, but it's too big 
for a mailing list anyway). Note that if I manually beam the notes 
(using '[' and ']') they look as they should (but it's something i'd 
prefer not to have to do).

Also any temporary workaround is appreciated as well as an explanation 
of what the heck is the error text suppoused to mean.

USING WinXP SP1, Athlon XP (Should that matter)
Lilypond 2.6.0 stable.

%%%%%%%%%%%%%%%%%% START CUT N PASTE %%%%%%%%%%%%%%%%%%
\version "2.6.0"
\score
{
 \layout
 {
  % No indent on first line
  indent = 0\mm
 }

 % start drums section
 \new DrumStaff <<
  % Polyphony mode (2 voices): setting up invisible voices
  \context DrumVoice = "1" { s1 }
  \context DrumVoice = "2" { s1 }
  \drummode {

\tempo 4=212

%%[SKIPPED]

\time 4/4

%% THIS IS A BOGUS MEASURE...
<< { \acciaccatura sn16 sn8 sn8 sn8 sn8 sn8 sn8 sn8 sn8  | } \\ {r1  | } >>

%%[SKIPPED]

%% THIS IS ANOTHER BOGUS MEASURE...

<< {r1  } \\ { \acciaccatura sn16 sn8 sn8 sn8 sn8 sn8 sn8 sn8 sn8 | } >>

% % % % % % % % % % % %
   % End of parsing. 657 lines processed
   }
 % end drumstaff
 >>
% end score
}

%%%%%%%%%%%%%%%%%% END CUT N PASTE %%%%%%%%%%%%%%%%%%

Thanks in advance and sorry for the big mail.

--

Ismael Cortés V.                      (address@hidden)
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