Re: Add 'subsequences' and 'permutations' to Data.List (ticket #1990)

Unfortunately, permutations5 has an error:

> *Main> permutations5 [1..3]
> [[1,2,3],[2,1,3],[3,2,1],[3,3,1],[3,2,2],[3,3,2]]
> *Main> permutations6 [1..3]
> [[1,2,3],[2,1,3],[3,2,1],[2,3,1],[3,1,2],[1,3,2]]