test for : msline mslinethickness="medium" (with mathsize="18pt")
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mstyle mathsize="18pt">
<mstack>
<mn>424</mn>
<msrow>
<mo>+</mo>
<none/>
<mn>33</mn>
</msrow>
<msline mslinethickness="medium"/>
</mstack>
</mstyle>
</math>
Author: Yecheng Gu
