docs: public ImmutableList<File> getExistingDirectories(String param) {
      final List<String> fileStrings = getStringList(param);
      final ImmutableList.Builder<File> ret = ImmutableList.builder();
  
      for (final String dirName : fileStrings) {
        final File dir = new File(dirName.trim());
        if (!dir.isDirectory()) {
          throw new ParameterValidationException(fullString(param), dirName,
              "path does not exist or is not a directory");
        }
        ret.add(dir);
      }
  
      return ret.build();
    }