Comment #1 on issue 2917 by [email protected]: Extend \keepWithTag to allow multiple tags
http://code.google.com/p/lilypond/issues/detail?id=2917
Since Nicolas' function works with either one argument or a list of arguments, replacing the current definition of \keepWithTag (without s) with his code seems the best way to go here. Other music functions concerning tags should be changed, too, of course.
