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fun intf base next 0 = base
...
                     (iota (x div c)))
end;

  datatype 'a option = Solution of 'a | Nothing;

  fun sflat ls = foldr (fn (Solution x, _) => Solution x
                         | (Nothing,v) => v) 
                       Nothing ls;
  fun smap _ Nothing = Nothing | smap f (Solution x) = Solution (f x)

local
  fun repeat x = intf [] (fn (_,ls) => x::ls)
...
         sflat (map (fn n => smap (fn x => (repeat c n) @ x) (change1 
+cs (x - n*c)))
                     (iota (x div c)))
end;

local val addup = foldr (fn (a,b) => a+b) 0
in
...
         addup (map (fn n => countchange cs (x - n*c))
                    (iota (x div c)))
end;