This pillar handles the doorway. It works out the point in the middle of the arch, 13 units from itself, and keeps it in its own +$09 to +$0B -- the velocity fields, which an arch never needs, borrowed because is_near_to compares +$09 to +$0B of one record with +$01 to +$03 of another. Then two passes over the player: chk_plyr_spec_near_arch, which marks it as allowed to leave the room if it is right in the doorway, and arch_nudge_to_centre, which nudges it towards the arch's centre line if it is anywhere near.

Arches in the north and south walls are mirrored (bit 6 of +$07) and lead along y; those in the east and west walls are not, and lead along x.