Generator for permutations, combinations, selections of a sequence (Python)

Python recipe 190465 by Ulrich Hoffmann (algorithms).

Permutations and combinations are often required in algorithms that do a complete search of the solution space. They are typically rather large so it's best not to compute them entirely but better to lazily generate them. This recipe uses Python 2.2 generators to create appropriate generator objects, that can be use for example as ranges in for loops.