let rec combinations acc size set = seq {

  match size, set with 

  | n, x::xs -> 

      if n > 0 then yield! combinations (x::acc) (n - 1) xs

      if n >= 0 then yield! combinations acc n xs 

  | 0, [] -> yield acc 

  | _, [] -> () }

​

combinations [] 3 [1 .. 5] |> Seq.iter (fun v -> printfn "%A" v)