functor BLOCKASDAG( structure B: BlockSig ):
sig structure G : GraphSig
    structure B : BlockSig
    sharing type G.Graph  = B.Block
        and type G.Vertex = B.Expn
end = 
struct
    structure B = B
    structure G =
    struct type Graph  = B.Block 
           type Vertex = B.Expn
           
           local open B in
           fun vertices (Block expns) = map #2 expns
           fun adj      _ e =
               case e of (x *** y) => [x, y]
                       | (x --- y) => [x, y]
                       | (x +++ y) => [x, y]
                       | (x /// y) => [x, y]
                       |  _        => []
           end
     end
end