sS regularLineBreaks(int charsPerLine, S s) {
  new StringBuilder buf;
  int n = l(s), j = 0;

  for i to n: {
    buf.append(s.charAt(i));
    if (++j >= charsPerLine) {
      buf.append('\n');
      j = 0;
    }
  }
  
  ret str(buf);
}