public function drush($command) {
    // @todo Set to silent if verbosity is less than very verbose.
    $bin = $this->getConfigValue('composer.bin');
    /** @var \Robo\Common\ProcessExecutor $process_executor */
    $drush_alias = $this->getConfigValue('drush.alias');
    $command_string = "'$bin/drush' @$drush_alias $command";

    // URIs do not work on remote drush aliases in Drush 9. Instead, it is
    // expected that the alias define the uri in its configuration.
    if ($drush_alias != 'self') {
      $command_string .= ' --uri=' . $this->getConfigValue('site');
    }

    $process_executor = Robo::process(new Process($command_string));

    return $process_executor->dir($this->getConfigValue('docroot'))
      ->interactive(FALSE)
      ->printOutput(TRUE)
      ->printMetadata(TRUE)
      ->setVerbosityThreshold(VerbosityThresholdInterface::VERBOSITY_VERY_VERBOSE);
  }