Calculating permutations in F#

Question

Inspired by this question and answer, how do I create a generic permutations algorithm in F#? Google doesn\'t give any useful answers to this.

EDIT: I provide my be

Answer 1

If you need distinct permuations (when the original set has duplicates), you can use this:

let rec insertions pre c post =
    seq {
        if List.length post = 0 then
            yield pre @ [c]
        else
            if List.forall (fun x->x<>c) post then
                yield pre@[c]@post
            yield! insertions (pre@[post.Head]) c post.Tail
        }

let rec permutations l =
    seq {
        if List.length l = 1 then
            yield l
        else
            let subperms = permutations l.Tail
            for sub in subperms do
                yield! insertions [] l.Head sub
        }

This is a straight-forward translation from this C# code. I am open to suggestions for a more functional look-and-feel.