}

  public void testHashCodeNull() {
    assertEquals(control(NK, 1).hashCode(), entry(NK, 1).hashCode());
    assertEquals(control("bar", NV).hashCode(), entry("bar", NV).hashCode());
    assertEquals(control(NK, NV).hashCode(), entry(NK, NV).hashCode());
  }

    /**
     * init graph
     */
    private void init(int nVertices, int nEdges) {
        int nV = nVertices;
        if (nVertices < 1) {
            nV = 1;
        }

        checkVertices(nV);

        int nE = nVertices;
        if (nEdges <= nV) {
            nE = 2 * nE;
        }

        checkEdges(nE);
    }

    private void checkVertices() {