I have an expression which can be simplified to a simpler one under the condition that two of its components are equal, and I'd like to put this fact into a single line equation.
Is there a generally-accepted notation for denoting that an equality is subject to a condition?
Some ways I can think of, but am not sure if they are universally understood (or possibly universally understood to mean something different...):
$a = \left. \frac{(c - d)e}{c^2 - d^2}= \frac{d}{c + d}\right|_{d=e} $
$a = \frac{(c - d)e}{c^2 - d^2}= \left(\frac{d}{c + d}\right)_{d=e} $
(Yes, I know that the middle term can already be simplified if d and e are not equal, but I don't care about that case — if I did, I could just use a curly brace conditional and list both options. I'm trying to keep it very short here.)
I think the method that would cause least confusion would be to put the "d=e" text below the equal sign, but I don't seem to be able to find a way to do that in LaTeX.