code: func ValidateInstallMode(mode InstallMode) error {
  	if mode == "" || mode == InstallModeCopy || mode == InstallModeSymlink {
  		return nil
  	}
  	return fmt.Errorf("invalid install mode %q", mode)
  }
docs: // ValidateInstallMode returns non nil if install mode is invalid.
  //
  // Valid modes are: "" (client will pick platform default), "copy"
  // (aka InstallModeCopy), "symlink" (aka InstallModeSymlink).