fun intf base next 0 = base
  | intf base next n = next (n, (intf base next (n-1)));
val iota = intf [0] (op ::);

local
      val append = foldl (op @) []
      fun repeat x = intf [] (fn (_,ls) => x::ls)
      fun rep2each x n ls = map (fn e => (repeat x n) @ e) ls
in
  fun change nil 0 = [[]]
    | change nil _ = []
    | change (c::cs) x =
         append (map (fn n => rep2each c n (change cs (x - n*c)))
                     (iota (x div c)))
end;