Sperner's proof {1,2,3} {1,2,3} {1,2} {1,3} {2,3} {1,2} {13} 23} 1 {2} 3} 1) 2} 3) 0 0Sperner’s proof ∅ {1} {2} {3} {1,2} {1,3} {2,3} {1,2,3} ∅ {1} {2} {3} {1,2} {1,3} {2,3} {1,2,3}