module for performing combinations and permutations on lists

 Combinatorics is the branch of mathematics studying the enumeration,
 combination, and permutation of sets of elements and the mathematical
 relations that characterize their properties. As a jumping off point,
 refer to:
 .
 http://mathworld.wolfram.com/Combinatorics.html
 .
 Math::Combinatorics provides a pure-perl implementation of nCk, nPk, and n!
 (combination, permutation, and factorial, respectively).