It is the aim of this article to survey some of the most important methods and results in this area. Of course by no means we can give a complete overview. Neverthelesswe hope that the selection of problems and methods might be helpful.
This article contains explicit combinatorial enumeration formulae as well as asymptotic results: in fact it turns out that in many problems of practical interest the latter kind of results is either the only one that can be achieved or even the preferable one for the interpretation of final results.
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