NR 3.3.2: \tagGroup

[Federico Bruni]

Hi all

If you scroll down to \tagGroup here:
http://lilypond.org/doc/v2.19/Documentation/notation/different-editions-from-one-source#using-tags

you find:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   \tagGroup #'(violinI violinII viola cello)

declares the respective tags as belonging to one tag group.

   \keepWithTag #'violinI …

will then only be concerned with tags from violinI’s tag group: any 
element of the included music that is tagged with one or more of tags 
from this set but not with violinI will get removed.

To any \keepWithTag command, only tags from the tag groups of the tags 
given in the command are visible.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I don't understand what this command does actually.
The last sentence is a bit confusing and there's no clear example 
showing the expected behaviour.

James, if it turns to be a documentation bug please hold on until 
Friday when I hope I've finished my translation (and review) of this 
chapter.

In the following example commenting/uncommenting \tagGroup does not 
change any behaviour.
What I'm missing?


\version "2.19.31"

music = \relative c'' {
 %\tagGroup #'(A B C D)
 \tag #'A { a4 a a a }
 \tag #'B { b4 b b b }
 \tag #'C { c4 c c c }
 \tag #'D { d4 d d d }
}

\new Staff {
 \keepWithTag #'A
 \music
}