mfenced tag bug ?

why can't why use the "<",  ">" in a mfenced tag?

like this:
<mfenced open='&lt;' close='&gt;'>
....
</mfenced>

i've  send an .xml example and a .jpg comparing amaya and mozilla
rendering in atachment.

Is it a bug ? where should it be reported?
btw can you tell me why amaya tells that there is a bug in the xml after
S0?
and why the validator doesn't recognize the  <math
xmlns="http://www.w3.org/1998/Math/MathML"> tag?

thanks in advance
joao