<!DOCTYPE not [
  <!ELEMENT not - - (Formula*)>
  <!NOTATION TeX SYSTEM "">
  <!NOTATION Eqn SYSTEM "">
  <!ELEMENT formula - O CDATA>
  <!ATTLIST formula notation NOTATION (TeX|Eqn) #CURRENT>
]>

<not>
<formula notation=TeX>
$$E = MC^2$$
</formula>
</not>
