“…Otherwise, the only Boolean elements of M i are 0 and 1, so by Proposition 2.4(11)(b), r i (0) − ⊙ r i (0) − = 0 or r i (0) − ⊙ r i (0) − = 1. The equality r i (0) − ⊙ r i (0) − = 0 implies that r i (0) − ≤ r i (0) consequently, r i (0) − = r i (0) ([DvZa3, (4.1)]) which means r i is strict and so M i is symmetric by [DvZa3,Thm 5.6]. If r i (0) − ⊙ r i (0) − = 1, then r i (0) = 0.…”