For example, categories populating the cell spanning abc in position (1, 3) can be built in two ways: either by combining entries in cell (1, 1) with entries in (2, 3) at midpoint m = 1; or by combining entries in (1, 2) and (3, 3) at midpoint m = 2.
cell (1, 2) is closed, hence there is only one midpoint at which (1, 3) can be built (m = 1).