Display:

$\begin{cases}0 & \text{if $x< 0$}\\ x & \text{otherwise}\end{cases}\left|x\right|=\begin{cases}-x & \text{if $x< 0$}\\ 0 & \text{if $x=0$}\\ x & \text{if $x> 0$}\end{cases}$

Source:

<piecewise>
     <piece>
          <cn> 0 </cn>
          <apply>
               <lt/>
               <ci> x </ci>
               <cn> 0 </cn>
          </apply>
     </piece>
     <otherwise>
          <ci> x </ci>
     </otherwise>
</piecewise>
<apply>
     <eq/>
     <apply>
          <abs/>
          <ci> x </ci>
     </apply>
     <piecewise>
          <piece>
               <apply>
                    <minus/>
                    <ci> x </ci>
               </apply>
               <apply>
                    <lt/>
                    <ci> x </ci>
                    <cn> 0 </cn>
               </apply>
          </piece>
          <piece>
               <cn> 0 </cn>
               <apply>
                    <eq/>
                    <ci> x </ci>
                    <cn> 0 </cn>
               </apply>
          </piece>
          <piece>
               <ci> x </ci>
               <apply>
                    <gt/>
                    <ci> x </ci>
                    <cn> 0 </cn>
               </apply>
          </piece>
     </piecewise>
</apply>