[Boost-users] BGL: why is the prim algorithm only useable for undirected graphs?

Hi all, in the documentation of prim_minimum_spanning_tree is mentioned "... solving the minimum spanning tree problem for an undirected graph with weighted edges ...". In my application I use this graph type (the graph type is bidirectionalS => not directed): typedef adjacency_list

BGraph;

This type modells the VertexListGraph and IncidenceGraph concepts, which are required by the prim function. If I call the prim_minimum_spanning_tree, I get reasonalble results. In literature, I can only find usage of Prims algorithm for undirected graphs, too. But I can see no reason, why it shouldn't work for directed graphs. Can you tell me more? Best regards Stefan Rauch