Question

Diagrams

Answer 1

$$
\require{AMScd}

$$

Answer 2

Preview works, but not posted, hmm...
$$
\begin{CD}
A @>a>> B\\
@V b V V{\large\circlearrowleft} @VV c V\\
C @>>d> D
\end{CD}
$$

Answer 3

http://www.jmilne.org/not/Mamscd.pdf

Answer 4

$$
\begin{CD}
A @<<< B @>>> C\\
@. @| @AAA\\
@. D @= E
\end{CD}
$$