code: public void setVariance (int dx, int dy)
      {
          if ((dx < 0) || (dy < 0) || ((dx == 0) && (dy == 0))) {
              throw new IllegalArgumentException(
                  "Variance values must be positive, and at least one must be non-zero.");
          } else {
              _dx = dx;
              _dy = dy;
          }
      }
docs: Set the variance of bobblin' in each direction.