<fn> and occurence

Hello, All.
This example is from manual
      <fn occurrence="infix"><mo>&PlusMinus;</mo></fn>

but in chap4_1.html said that 'occurrence' attribute may be used only
with <declare>. 
In this case we may write just <fn><mo>&PlusMinus;</mo></fn> because
PlusMinus is listed in dictionary. 
So, we may accept the 'occurrence' attribute in <fn>.
But if I want to write n@ in content murkup (n factorial, but for '!'
we use '@') I will fail because there is no predefined value 'postfix'
for the occurrence attribute.

And second.
What should we render if we see
Should we use the default value occurrence="Function-model" or use
'form' of operator?

I have many more questions on this topic.
May be someone have FAQ on it? ;-)

Get your free @yahoo.com address at http://mail.yahoo.com