mfencedAempty.test

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

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mfenced separators="" open="" close=" ">
    <mi>x</mi> 
    <mi>y</mi> 
  </mfenced> </math>
>>> native
[ EGrouped [ EIdentifier "x" , EIdentifier "y" ]
, EMathOperator ""
]