For example:
$f(x)=f(x_0)+f'(x_0)(x-x_0)+\frac{1}{2!}f''(x_0)(x-x_0)^2+\dots=\exp\left((x-x_0)\frac{\mathrm{d}}{\mathrm{d}x}\right)f(x)\Big|_{x_0}$
I don't know how to write the $\Big|_{x_0}$ to the front … Is this thing an operator? How to write it down more elegantly?