docs: public static DoubleBinaryOperator dx(DoubleBinaryOperator f)
      {
          return (x,y)->
          {
              double h = x != 0.0 ? SQRT_EPSILON*x : SQRT_EPSILON;
              double h2 = 2.0*h;
              double y1 = -f.applyAsDouble(x+h2, y);
              double y2 = 8.0*f.applyAsDouble(x+h, y);
              double y3 = -8.0*f.applyAsDouble(x-h, y);
              double y4 = f.applyAsDouble(x-h2, y);
              return (y1+y2+y3+y4)/(12.0*h);
          };
      }