public @Nullable Set<String> transform(@Nullable Collection<String> words) {
    if (words == null || words.isEmpty()) {
      return null;
    }
    Set<String> result = new HashSet<>();
    for (String word : words) {
      String transformed = transform(word);
      if (transformed != null) {
        result.add(transformed);
      }
    }
    return result;
  }
public @Nullable Set<String> transform(@Nullable Collection<String> words)