<?xml version="1.0" encoding="UTF-8"?>
<math xmlns="http://www.w3.org/1998/Math/MathML">
   <mrow xmlns="">
      <mo fence="true"
          form="prefix"
          lspace="0em"
          rspace="0.3em"
          stretchy="true">{</mo>
      <mn>3</mn>
      <mi>k</mi>
      <mo>+</mo>
      <mn>4</mn>\<mo fence="true"
          form="postfix"
          lspace="0.3em"
          rspace="0em"
          stretchy="true">}</mo>
      <mo>⊂</mo>
      <mi>ℭ</mi>
   </mrow>
</math>
