1. Aho, A., Hopcroft, J., and Ullman, J. (1974)The Design and Analysis of Computer Algorithms, Addison-Wesley, Reading, Mass.
2. Brightwell, G. and Winkler, P. (1991) Counting linear extensions,Order 8, 225?242.
3. Ehrlich, G. (1973) Loopless algorithms for generating permutations, combinations, and other combinatorial configurations,J. Assoc. Comput. Mach. 20, 500?513.
4. Even, S. (1973)Algorithmic Combinatorics, The Macmillan Company, New York.
5. Gray, F. (1953) Pulse code communication,U.S. Patent 2632058.