1. I thought I'd make a new thread for some snippets of more "advanced" LaTeX codes that are not covered by the basic LaTeX tutorial.

For example:
$\displaystyle \enclose{circle}{x}$
\enclose{circle}{x}

$\displaystyle A \overset{u}{=} B$
A \overset{u}{=} B

$\displaystyle A \overset{u}{ \rightarrow} B$
A \overset{u}{ \rightarrow } B

$\displaystyle A \underset{d}{ \rightarrow } B$
\underset{b}{ \rightarrow } B

$\displaystyle A \overset{u}{\underset{d} \rightleftarrows} X$
A \overset{u}{ \underset{d} \rightleftarrows } X

(I got the first one from Klaas van Aarsen. Thanks!)

-Dan

2.