- Compute the column of the right-items. This is a big function,
- since RIGHT-ITEMS may span more columns (eg. if a clef is inserted,
- this will add a new column to RIGHT-ITEMS. Here we look at the
- columns, and return the left-most. If there are multiple columns, we
- prune RIGHT-ITEMS.
-
- 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.