“…If S = I \ J is a skew ideal, then a point H ∈ S such that S \ H is still a skew ideal is called a corner of S. We call it a lower corner if J ∪ H is an ideal, and an upper corner if I \ H is an ideal. For example, in Figure 11, with S the set of labelled points, the lower corners are (1, 3), (2, 2), (3, 1) and the upper corners are (1,4), (3,3), (3,2), (4,1).…”