functor GRAPH(  type Vertex
           structure D:DictSig 
                 and VS:SetSig 
             sharing type Vertex = VS.Item = AS.Item = D.Key 
                 and type Vertex list = D.Item) =
struct
   type Vertex = Vertex
   structure VSet = VS

   type Graph = VS.Set * (Vertex list) D.Dict
   fun vertices (v,a) = v
   and adj (v,a) x = D.lookup a x
end;