Re: Musicxml2ly: Fix incorrect conversion of Minor Chords (issue 305700043 by address@hidden)

[Johan Vromans]

> > The bottom line is: What is required in chord c:blah so that .NN can  
> > be added as a pure addition. It is unfortunate that c:.13 is invalid  
> > syntax.

Since MusicXML additions are pure additions, would it be safe to turn these
into (addNN)?

E.g. C + 13 => c:(add13)

Even though that will turn a friendly C13 (user input) via C + 7 + 9 + 11 +
13 (MusicXML) into an ugly c:(add7)(add9)(add11)(add13) (LilyPond)...

Otherwise, I'd go for the origional idea of parsing for a preceding digit,
optionally followed by '+' or '-' (if that would cover all).