void POVWriterNodeVisitor::pushStateSet( const StateSet *ss )
{
   if( ss ) {

      // merge state set
      StateSet *mergedSS = new StateSet(
            *_stateSetStack.top().get(), CopyOp::SHALLOW_COPY );
      mergedSS->merge( *ss );

      // push state set
      _stateSetStack.push( mergedSS );

   }
}