def install?
      s = super
      return s unless s
      fail InstallFailed, 'No file source path specified.' if source_path.nil?
      fail InstallFailed, "Source path does not exist: #{source}" unless File.exist? source_path
      true
    end
def install?
      s = super
      return s unless s
      fail InstallFailed, 'No file source path specified.' if source_path.nil?
      fail InstallFailed, "Source path does not exist: #{source}" unless File.exist? source_path
      true
    end
(see Unit#install?)