[graph-tool] Mark all reachable Nodes

Hi, I need to find a way to label all reachable nodes in a directed graph starting at a specific node. Is there a short way to do this? Currently I use this: ``` graph = get_graph() is_reachable = graph.new_vp("bool") start_vertex = get_start_vertex() tc = gt.transitive_closure(graph) is_reachable[start_vertex] = True for node in tc.vertex(start_vertex).out_neighbors(): is_reachable[graph.vertex(node)] = True ``` This works, but seems quite inefficient, since I don't need the full transitive closure (although I have multiple such start vertexes). Another possibility should be a raw DFS but this needs far more LOC, since it needs to stop when it backtracks and therefore needs a custom visitor. Gerion