let rec combinations n l =
        match n, l with
        | 0, _ -> [[]]
        | _, [] -> []
        | k, (x::xs) -> List.map ((@) [x]) (combinations (k-1) xs) @ combinations k xs
 
let rec cross xs = seq {
        match xs with 
        | h :: t -> for e in t do yield h, e        
                    yield! cross t
        | [] -> ()
        }
 
let solve n (lst : int list) = 
        seq { for e in 1..n do yield! combinations e lst }
        |> Seq.groupBy List.sum
        |> Seq.collect (snd >> Seq.toList >> cross)
 
solve 7 [1..14] 
|> Seq.last
|> printfn "%A"

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