rec-bvar1

W3C sample	WIRIS
	$\frac{d^{2}x^4}{dx^2}$

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <apply>
    <diff/>
    <bvar>
      <ci>x</ci>
      <degree>
        <cn>2</cn>
      </degree>
    </bvar>
    <apply>
      <power/>
      <ci>x</ci>
      <cn>4</cn>
    </apply>
  </apply>
</math>

Author: MathML 2 Recommendation