public function drush($command) {
    // Clear out options associated with previous drush command.
    $this->setOptionsForLastCommand();

    if (!$this->defaultsInitialized) {
      $this->init();
    }

    if (is_array($command)) {
      $command = implode(' ', array_filter($command));
    }

    $this->commands[] = trim($command);
    return $this;
  }