(* Return a function that maps list items using a given function lambda *)
fun map(lambda) = fn ([]) => []
                   | (x::[]) => [lambda(x)]
                   | (xs) => lambda(hd(xs)) :: map(lambda)(tl(xs));

(* Return a list of all k element sublists of xs *)
fun choose(0, xs) = [[]]
  | choose(1, xs) = map(fn x => [x])(xs)
  | choose(k, xs) = if k > length(xs) then []
    else if k = length(xs) then [xs]
    (* The previous case of tl(xs) as well as all the new sublists with
       the head *)
    else choose(k, tl(xs)) @ map(fn y => hd(xs) :: y)(choose(k-1, tl(xs)));