local
  fun repeat x = intf [] (fn (_,ls) => x::ls)
in
  fun change1 nil 0 = Solution []
    | change1 nil _ = Nothing
    | change1 (c::cs) x =
         sflat (map (fn n => smap (fn x => (repeat c n) @ x) (change1 cs (x - n*c)))
                     (iota (x div c)))
end;