public function compile(ExpressionCompiler $compiler, ExpressionInterface $expr)
 {
     if (isset($this->serviceMap[$expr->name])) {
         $compiler->write("\$context['container']->get('{$this->serviceMap[$expr->name]}'");
         if ($expr->allowNull) {
             $compiler->write(", " . ContainerInterface::NULL_ON_INVALID_REFERENCE);
         }
         $compiler->write(")");
         return;
     }
     if (isset($this->parameterMap[$expr->name])) {
         $compiler->write("\$context['container']->getParameter('{$this->parameterMap[$expr->name]}')");
         return;
     }
     parent::compile($compiler, $expr);
 }
 protected function isKnown($variable)
 {
     return isset($this->serviceMap[$variable]) || isset($this->parameterMap[$variable]) || parent::isKnown($variable);
 }