xn 2  pxn 4  palbits 1 ppalbits 2
  M #      0 = 1001   :   2
    Members:
    ..
    ab

sigmas with   2 members: 1
  M #      1 = 1111   :   4
    Members:
    ..
    a.
    .b
    ab

sigmas with   4 members: 1

n = 2, sigma count 2.