docs: public static double integral(DoubleUnaryOperator f, double x1, double x2, int points)
      {
          double delta = (x2-x1)/points;
          double delta2 = delta/2.0;
          double sum = 0;
          double y1 = f.applyAsDouble(x1);
          double y2;
          for (int ii=1;ii<=points;ii++)
          {
              x1 += delta;
              y2 = f.applyAsDouble(x1);
              sum += (y1+y2)*delta2;
              y1 = y2;
          }
          return sum;
      }