[Tex/LaTex] Underscripted Equals Symbol

Question

Is there a non-crazy way I can implement a simple = sign with a condition/scope written under it?
In particular, (expr1) = (x->a) (expr2), where (x->a) is written like in a limit, under the equals sign?

Answer 1

Like this?

\documentclass{article}
\newcommand\underrel[2]{\mathrel{\mathop{#2}\limits_{#1}}}
\begin{document}
$x \underrel{x\to a}{=} a$
\end{document}

You could also add an optional argument which enables you to let the underset content bleed to the left and right.

\documentclass{article}
\usepackage{mathtools}
\newcommand\underrel[3][]{\mathrel{\mathop{#3}\limits_{%
      \ifx c#1\relax\mathclap{#2}\else#2\fi}}}
\begin{document}
$
x \underrel{x\to a}{=} a
\quad
x \underrel[c]{x\to a}{=} a
$
\end{document}