functor DAGOPT2FLOWOPT( structure F : FlowSig
                        val blockOpt: F.B.Block -> F.B.Block ) :
        sig structure OF : FlowSig
            val flowOpt  : F.G.Graph -> OF.G.Graph
        end =
struct
exception Lookup
fun lookup [] _ = raise Lookup
  | lookup ((k,e) :: t) k' = if k = k' then e else lookup t k'

structure OF =
struct
   structure B = F.B
   structure C = F.C
   structure G =
   struct
      type Vertex = int
      datatype Graph = 
         FG of( Vertex 
                * ( Vertex list * (B.Block * C.Cond) )
              ) list

      fun vertices (FG g) = map #1 g
      fun adj  (FG g) v   = #1(lookup g v)
   end
   fun data (G.FG g) v        = #2(lookup g v)
end

   fun opt (b,c) = (blockOpt b, c)

   fun flowOpt g =
   OF.G.FG(map (fn j => (j, (F.G.adj g j, opt (F.data g j)))) (F.G.vertices g))
end