title: Velocity: Is a any way to check if variable is defined
body: <p>I want to include one template <code>nested</code> into others <code>cont1</code>, <code>cont2</code>, <code>cont3</code>. 
  And nested template should be hide one specific control for cont1 only.
  Before inclusion into <code>cont1</code> I would like to assign value to some flag variable <code>$hideMyControl</code>. </p>
  
  <p>And inside nested template I would like to check if <code>$hideMyControl</code> is assigned value.</p>
  
  <p>How to perform such check?</p>