[boost] [BGL] clear_out_edges and MutableIncidenceGraph

Hi, adjacency_list implements clear_out_edges, but it's not required by any of the graph concepts: http://www.boost.org/doc/libs/1_46_1/libs/graph/doc/graph_concepts.html Could it be added to MutableIncidenceGraph? Actually, now that I check, I don't see MutableIncidenceGraph on that page either, although it is in graph_concepts.hpp. MutableIncidenceGraph requires remove_edge and remove_out_edge_if. clear_out_edges would fit nicely in here, and clear_in_edges in MutableBidirectionalGraph. Cheers, Shaun