Fixed a type error in flow_graph:add_edge/2.

src/flow_graph.erl

@@ -134,7 +134,7 @@ out_edges(#flow_graph{graph=G}, #vertex{ref=R}) ->
 
 add_edge(FG = #flow_graph{graph=G}, E = #edge{from=From, to=To}) ->
     Add = fun(Edges) -> [E|Edges] end,
-    FG#flow_graph{graph=?DICT:update(From, Add, [E], add_vertex(G,To))}.
+    add_vertex(FG#flow_graph{graph=?DICT:update(From, Add, [E], G)}, To).
 
 add_edge_list(FG = #flow_graph{}, Edges) ->
     lists:foldl(fun(E, Acc) -> add_edge(Acc, E) end, FG, Edges).