code:     private void checkParams()
      {
          if (vi == null)
          {
              throw new IllegalArgumentException("no layers defined.");
          }
          if (vi.length > 1)
          {
              for (int i = 0; i < vi.length - 1; i++)
              {
                  if (vi[i] >= vi[i + 1])
                  {
                      throw new IllegalArgumentException(
                          "v[i] has to be smaller than v[i+1]");
                  }
              }
          }
          else
          {
              throw new IllegalArgumentException(
                  "Rainbow needs at least 1 layer, such that v1 < v2.");
          }
      }
  
      /**
       * Getter for the number of layers
       *
       * @return the number of layers
       */
  