// Distribute an element y over a list:

// {x1,..,xn} --> {y,x1,..,xn}, {x1,y,x2,..,xn}, .. , {x1,..,xn,y}

let distrib y L =

    let rec aux pre post = seq {

        match post with

        | [] -> yield (L @ [y])

        | h::t -> yield (pre @ y::post)

                  yield! aux (pre @ [h]) t }

    aux [] L

​

// All permutations of a single list = the head of a list distributed

// over all permutations of its tail

let rec getAllPerms = function

    | [] -> Seq.singleton []

    | h::t -> getAllPerms t |> Seq.collect (distrib h)

​

getAllPerms [1;2;3] |> Seq.toList |> printfn "%A"

getAllPerms [1;2;3;4;5] |> Seq.toList |> printfn "%A"

​