1. L. H.Haines (1969) On free monoids partially ordered by embedding, J. Comb. Theory 6, 94?98.
2. G.Higman (1952) Ordering by divisibility in abstract algebras, Proc. London Math. Soc. 2, 225?336.
3. H. Jurgensen and G. Thierrin (1985) Infix codes, in Proc. Conf. Computing Sc., Gyor (Hungary) (to appear).
4. G.Lallement (1979) Semigroups and Combinatorial Applications, Wiley-Interscience, New York.
5. H.Prodinger and G.Thierrin (1983) Towards a general concept of hypercodes, J. Info. Optim. Sci. 4, 254?268.