Poor error recovery in grace-beaming mismatch

[Hans Aberg]

This problem arises when generating .ly files using "abc2ly -- 
beams=None" (version 2.11.34), which generate mismatch beaming "["  
and \grace{...}. When LilyPond compiles this file, the grace notes  
will be replaced by 1/4 pointing the wrong way, and will further also  
on grace notes further down which are syntactically correct. So  
LilyPond has not only a a problem of not being able to generate a  
decent output given the syntax error, but also cannot recover  
properly from the error.

An ABC source file

T:abc2ly grace note bug
M:C
L:1/8
Q:1/4=120
K:E Dorian
{ga}gdgb {a}g>d g<a|

compiled using "abc2ly --beams=None" generates code of the form
...
\grace {    gis''8[    a''8  }   gis''8    dis''8
gis''8    b''8]  \grace {    a''8  }   gis''8.[    dis''16]    gis''16[
a''8.]
...

Here, the first "[" within the \grace{...} construct does not have a  
closing "]". However, the code is after is closing "]" correct. But  
also the second \grace{...} gets a funny putout. It looks as in the  
attachment.

  Hans Aberg