Dwight Duffus Bill Sands Norbert Sauer Robert E. Woodrow Two-colouring all two-element maximal antichains. 109-116 1991 57 J. Comb. Theory, Ser. A 1 http://dx.doi.org/10.1016/0097-3165(91)90009-6 db/journals/jct/jcta57.html#DuffusSSW91