theorem SquaredBottom:
  Bottom (B squared-latt) = [Bottom B, Bottom B]
  proof
    [Bottom B, Bottom B] in B squared; then
    Bottom [:B,B:] in B squared by FILTER_1:42; then
    Bottom latt ([:B,B:], B squared) = Bottom [:B,B:] by ThB;
    hence thesis by FILTER_1:42;
  end;
