docs: @javax.annotation.Nonnull
    public static List<double[]> op(@javax.annotation.Nonnull final List<double[]> a, @javax.annotation.Nonnull final DoubleUnaryOperator fn) {
      @javax.annotation.Nonnull final ArrayList<double[]> list = new ArrayList<>();
      for (int i = 0; i < a.size(); i++) {
        @javax.annotation.Nonnull final double[] c = new double[a.get(i).length];
        for (int j = 0; j < a.get(i).length; j++) {
          c[j] = fn.applyAsDouble(a.get(i)[j]);
        }
        list.add(c);
      }
      return list;
    }