I'm typing a document in the typsetting language LaTeX.  How do I make
a vertical bar next to a derivative fraction that means "evaluate at"?
 I want to write something like \frac{df}{dx} |_{x=0}, but this (and
vert) makes the vertical bar much too short.  I think the vertical bar
is supposed to be as tall as the fraction.  \right | would make it the
right size, but I need a matching \left |, and I don't want that.

thanks