(*  ML ASSESSED EXERCISES. TICK 3* SUBMISSION *)

fun choose(k,xs)=
  let
    fun f(_,0,asub,subs)=asub::subs
      | f([],_,_,subs)=subs
      | f(x::xs,k,asub,subs)=
          f(xs,k-1,asub @ [x],f(xs,k,asub,subs))
  in
    f(xs,k,[],[])
  end;