## Rectangular diagrams ##

AMScd diagrams must start with 
\require{AMScd} between dollar signs followed by

 $\require{AMScd}$
 \begin{CD}
 A @>a>> B\\
 @V b V V\# @VV c V\\
 C @>>d> D
 \end{CD}

to get this diagram:
$\require{AMScd}$
\begin{CD}
A @>a>> B\\
@V b V V\# @VV c V\\
C @>>d> D
\end{CD}

`@>>>` is used for arrow right

`@<<<` is used for arrow left

`@VVV` is used for arrow down

`@AAA` is used for arrow up

`@=` is used for horizontal double line

`@|` is used for vertical double line

`@.` is used for no arrow

Another example:


 \begin{CD}
 A @>>> B @>{\text{very long label}}>> C \\
 @. @AAA @| \\
 D @= E @<<< F
 \end{CD}


\begin{CD}
A @>>> B @>{\text{very long label}}>> C \\
@. @AAA @| \\
D @= E @<<< F
\end{CD}

Long labels increase the length of the arrow and in this version also automatically increase corresponding arrows.