Where Can I Learn Advanced LaTeX Codes?

[topsquark]

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:
[math]\enclose{circle}{x}[/math]
\enclose{circle}{x}

[math]A \overset{u}{=} B[/math]
A \overset{u}{=} B

[math]A \overset{u}{ \rightarrow} B[/math]
A \overset{u}{ \rightarrow } B

[math]A \underset{d}{ \rightarrow } B[/math]
\underset{b}{ \rightarrow } B

[math]A \overset{u}{\underset{d} \rightleftarrows} X[/math]
A \overset{u}{ \underset{d} \rightleftarrows } X

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

-Dan