<?xml version="1.0"?>
<!DOCTYPE set SYSTEM "mathml.dtd"> 
<set>
        <bvar>  <ci>x</ci>  </bvar>  
        <bvar>  <ci>y</ci>  </bvar>  
        <bvar>  <ci>z</ci>  </bvar>  
        <condition>
         <reln> <eq/>
          <apply> <plus/>
            <apply> <power/> <ci>x</ci> <ci>2</ci> </apply>
            <apply> <power/> <ci>y</ci> <ci>2</ci> </apply>
            <apply> <power/> <ci>z</ci> <ci>2</ci> </apply>
          </apply>
          <cn>1</cn>
         </reln>
      </condition>
      </set>

