+
+ If we end up pruning, we add a left-neighbor to every column that
+ gets pruned. This ensures that loose columns in cross-staff music
+ do indeed get marked as loose. The problem situation is when a voice
+ passes from staff 1 to staff 2 and a clef appears later on in staff 1.
+ Then the NoteSpacing attached to the last note in staff 1 has two
+ right-items: one pointing to the next note in staff 2 and one pointing
+ to the clef. We will prune the clef right-item here and, unless we add
+ a left-neighbor to the clef, it won't get marked as loose.