I used P2FA to insert all of these boundaries. For some reason, there is an invisible boundary. You can see this is part of the selection. I would like to hand-correct the interval labelled B (on tier 1), but I'm unable to do so. I tried dragging the left boundary of B to the left, but it doesn't let me. I think it has something to do with this invisible boundary.
Thanks. I hope this makes sense.