<?xml version="1.0" encoding="UTF-8"?>
<math xmlns="http://www.w3.org/1998/Math/MathML">
   <mfenced close="}" open="{">
      <mrow>
         <mn>3</mn>
         <mi>k</mi>
         <mo>+</mo>
         <mn>4</mn>
         <mi> </mi>
      </mrow>
   </mfenced>
   <mo>⊂</mo>
   <mi mathvariant="fraktur">C</mi>
</math>
