# \mathclap not centering

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?

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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.

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