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