docs: private double preNorm(NumberVector v, final int start, final int end) {
      double agg = 0.;
      for(int d = start; d < end; d++) {
        final double xd = v.doubleValue(d);
        final double delta = xd >= 0. ? xd : -xd;
        agg += MathUtil.powi(delta, intp);
      }
      return agg;
    }