1. B. I. Bayoumi, M. H. El-Zahar, and S. M. Khamis, Counting two-dimensional posets, preprint.
2. L.Beineke and R. L.Hemminger (1978) Line graphs and line digraphs in L.Beineke and R. J.Wilson (eds.),Selected Topics in Graph Theory, Academic Press, New York.
3. M.El-Zahar (1987) Enumeration of ordered sets, in I.Rival (ed.),Algorithms and Order, Kluwer Academic Publishers, Dordrecht, pp. 327?352.
4. P.Grillet (1969) Maximal chains and antichains,Fund. Math. 65, 157?167.
5. M.Habib and R.M�hring (1987) On some complexity properties ofN-free posets and posets with bounded decomposition diameter,Discrete Math. 63, 157?182.