data/mathtoweb/244.mml
<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo rspace="0.3em" lspace="0em" stretchy="true" fence="true" form="prefix">{</mo><mn>3</mn><mi>k</mi><mo>+</mo><mn>4</mn>\<mo rspace="0em" lspace="0.3em" stretchy="true" fence="true" form="postfix">}</mo><mo>⊂</mo><mi>ℭ</mi></mrow></math>