def install?
      s = super
      return s unless s
      fail InstallFailed, 'No source file specified.' if source_path.nil?
      fail InstallFailed, 'No destination specified.' if destination_path.nil?
      true
    end
def install?
      s = super
      return s unless s
      fail InstallFailed, 'No source file specified.' if source_path.nil?
      fail InstallFailed, 'No destination specified.' if destination_path.nil?
      true
    end
(see Unit#install?)