emptyContent1.test

<<< mml
<?xml version="1.0" encoding="UTF-8"?>

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <msub>
    <mn></mn> 
    <mo></mo> 
  </msub> 
</math>
>>> native
[ ESub (ENumber "") (EMathOperator "") ]