static S[][] wordsMatrix(Collection<S> lines) {
  LL<S> l = map words(lines);
  int rows = l(l), cols = maxLength(l);
  S[][] matrix = new S[rows][cols];
  for i to rows: {
    L<S> words = l.get(i);
    for j to cols:
      matrix[i][j] = j < l(words) ? words.get(j) : "";
  }
  ret matrix;
}