Bugreport: Accidental Modern bug when partial bar first

[Donald Axel]

   This is an edited reposting of a bugreport. I am reposting so that
it is not forgotten amidst a long thread.

   The bug is a minor (small) bug concerning accidental-rules.

   After reading documentation (see link below) and experimenting with
Lily-2.8.5 I think Marco Gusy has discovered a bug. If the partial bar
has a natural C then the "modern" rules are ignored, no natural sign
appears in the attached example with a partial bar. The partial bar
natural C should not influence the application of the "modern"-rule
in the next full bar.

   The attached PNGs show the bug with "modern" style.

   As there has been some discussion there may be need for a very
explicit explanation:

   The "modern"-rule says that "[...] temporary accidentals are also
cancelled in other octaves."

   Therefore the bars before the temporary accidentals should not
influence the "modern"-rule.

   (Same applies to modern-cautionary which works differently
from the documentation when there is a partial bar first.)

   Using the documentation example:

% ****************************************************************
% ly snippet contents follows:
% ****************************************************************
#(set-accidental-style 'modern-cautionary)
cis' c'' cis'2 | c'' c'
% ***end***

   Adding natural C as partial first bar changes the subsequent bar:

% ****************************************************************
% ly snippet contents follows:
% ****************************************************************
#(set-accidental-style 'modern) % applies to modern-cautionary (too).
\partial 4
         c''   |
cis' c'' cis'2 | c'' c'
% ***end***

http://lilypond.org/doc/v2.8/Documentation/user/lilypond/Automatic-accidentals.html#Automatic-accidentals

docu-acc-modern.png
Description: PNG image

docu-acc-modern-pre4.png
Description: PNG image