signature GRAPH =
sig

  type 'a graph = ('a, 'a) Rel.relation

  val depthFirst : ''a * ''a graph -> ''a Set.set
end;


structure Graph :> GRAPH =
struct

  type 'a graph = ('a, 'a) Rel.relation

  (* val depthFirst : ''a * ''a graph -> ''a Set.set *)
  fun depthFirst (x, g)=
    let
      (* val df' : ''a list * ''a Set.set -> ''a Set.set *)
       fun df' (nil, visited) = visited
         | df' (x::xs, visited) =
            if Set.setMember (x, visited)
               then df' (xs, visited)
               else
                 let
                   val vs = Rel.relImage (x, g)
                 in
                   df' ((Set.setToList vs) @ xs, Set.setInsert (x, visited))
                 end
    in
      df' ([x], Set.emptySet)
    end
end;