Comment #4 on issue 2571 by [email protected]: tuplet bracket starts to late when the expression starts with a spacer
http://code.google.com/p/lilypond/issues/detail?id=2571
I think there are not really such anchors, at least not in the form of grobs from the spacer rest. Possibly one could include currentCommandColumn (?) at the start and end of the tuplets.
