public function execute(Sandbox $sandbox)
  {
    $language = new ExpressionLanguage($sandbox);
    Container::getLogger()->info("Evaluating expression: " . $language->compile($this->expression));
    try {
      if ($return = $language->evaluate($this->expression)) {
        Container::getLogger()->debug(__CLASS__ . ": Expression PASSED: $return");
        return $return;
      }
    }
    catch (\Exception $e) {
      Container::getLogger()->warning($e->getMessage());
    }
    Container::getLogger()->debug(__CLASS__ . ": Expression FAILED.");

    // Execute the on fail behaviour.
    throw new DependencyException($this);
  }