Used by the routines at ADJ_DZ_FOR_OUT_OF_BOUNDS, ADJ_DU_FOR_OBJ_INTERSECT, ADJ_DV_FOR_OBJ_INTERSECT, ADJ_DZ_FOR_OBJ_INTERSECT, ADJ_DU_FOR_OUT_OF_BOUNDS and ADJ_DV_FOR_OUT_OF_BOUNDS.

Moves A one step towards zero -- down if positive, up if negative -- and leaves Z set if nothing is left. Every loop in the collision code uses it to cut a move short a unit at a time and try again.