code: protected function optimizePrintNode(Twig_NodeInterface $node, Twig_Environment $env)
      {
          if (!$node instanceof Twig_Node_Print) {
              return $node;
          }
  
          $exprNode = $node->getNode('expr');
          if (
              $exprNode instanceof Twig_Node_Expression_BlockReference ||
              $exprNode instanceof Twig_Node_Expression_Parent
          ) {
              $exprNode->setAttribute('output', true);
  
              return $exprNode;
          }
  
          return $node;
      }
docs: Optimizes print nodes.
  
  It replaces:
  
  * "echo $this->render(Parent)Block()" with "$this->display(Parent)Block()"
  
  @return Twig_NodeInterface