Adds a visitor.
Twig_NodeVisitorInterface $visitor A Twig_NodeVisitorInterface instance:
public function addVisitor(Twig_NodeVisitorInterface $visitor) {
if (!isset($this->visitors[$visitor
->getPriority()])) {
$this->visitors[$visitor
->getPriority()] = array();
}
$this->visitors[$visitor
->getPriority()][] = $visitor;
}