Display:

$\frac{d^{2}x^4}{dx^2}$

Source:

<apply>
     <diff/>
     <bvar>
          <ci> x </ci>
          <degree>
               <cn> 2 </cn>
          </degree>
     </bvar>
     <apply>
          <power/>
          <ci> x </ci>
          <cn> 4 </cn>
     </apply>
</apply>