code: public static <V> Set<V> getIncomingVertices(DirectedGraph<V, DefaultEdge> graph, V target) {
          Set<DefaultEdge> edges = graph.incomingEdgesOf(target);
          Set<V> sources = new LinkedHashSet<V>();
          for (DefaultEdge edge : edges) {
              sources.add(graph.getEdgeSource(edge));
          }
          return sources;
      }
docs: Fetch all of the dependents of the given target vertex
  @return mutable snapshot of the source vertices of all incoming edges