\mathclap not centering

Question

I'm trying to stack two symbols on top of each other (not \stackrel) using \mathclap in the mathtools package, but the symbols are not centering:

\mathclap{\rightleftharpoons}\circ

How do I get the circle in the center of the arrows?

Answer 1

This is a job for \ooalign:

\newcommand{\circrlh}{\mathrel{\mathpalette\docircrlh\relax}}
\newcommand{\docircrlh}[2]{%
  \ooalign{$#1\rightleftharpoons$\cr\hfil$#1\circ$\hfil\cr}}

Now with

$a\circrlh b_{\circrlh}$

you get the following result.

See this answer for a quick course on \ooalign