[O] bug in odt export via mathml of equations containing '&'

[Myles English]

If an org file contains a latex equation with a '&' in it then when it is
exported to odt it makes dodgy xml.  Unzipping the odt, opening the
content.xml and doing M-x rng-first-error gives the message:

`&' that is not markup must be entered as `&'

To reproduce, insert this:

\begin{equation}
\delta_{mn} = 
 \begin{cases}
  1& \text{if $n=m$}\\
  0& \text{if $n\nem$}
 \end{cases}
\end{equation}

(which I got from here
http://www.mathtoweb.com/cgi-bin/mathtoweb_users_guide.pl , search for
'cases')

into the file math-to-web-with-plastex.org in this post:
http://permalink.gmane.org/gmane.emacs.orgmode/48815 and export as per
instructions.

There may be a similar error with equations containing '<', '>'.

Myles