Display:

$\int_{x\in D} f(x)\,d x$

Source:

<apply>
     <int/>
     <bvar>
          <ci> x </ci>
     </bvar>
     <condition>
          <apply>
               <in/>
               <ci> x </ci>
               <ci> D </ci>
          </apply>
     </condition>
     <apply>
          <ci type="fn"> f </ci>
          <ci> x </ci>
     </apply>
</apply>