[OS X TeX] Commutative Diagrams

[Alain Schremmer]

I am trying to typeset

\[ \begin{CD}
1 @>>> 2 \\
@VVV @VVV \\
3 @>>> 4 \\
\end{CD} \]

which I believe to be a simpler version of the example in Companion 2ed, 
page 468. (I have installed amscd with all my other .sty and entered 
\usepackage{amscd} in the preamble.)

The console complains about

!Misplaced \noalign.
\math at cr@@ … \iffalse }
etc
but, as soon as I try to copy it, this turns into

!Missing $ inserted
etc

If I type r and enter I get

(That makes 100 errors, please try again,)
! ==> Fatal error
etc

Would anyone have an idea of what I am doing wrong?

Regards
--schremmer
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