code: public static <V> void addAllVertices(DirectedGraph<V, DefaultEdge> graph, Set<V> vertices) {
          // add all of the new vertices to prep for linking
          for (V vertex : vertices) {
              graph.addVertex(vertex);
          }
      }
docs: Add all of the vertices to the graph without any edges. If the the graph
  already contains any one of the vertices, that vertex is not added.
  @param graph graph to be mutated
  @param vertices vertices to add