Simply enclose the fraction in a pair of vertical bars and delete the left one.
Basile
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[excusez la concision]

> Le 13 sept. 2018 à 20:39, Jorge Fernández de Cossío Díaz <address@hidden> a écrit :
>
> How can I obtain a derivative evaluated at a point, such as:
> <image.png>
>
> I don't know how to print the long bar on the right (without a matching long bar on the left) with the subscripted x0.