sS regularSpecialLineBreaks(int charsPerLine, S lineBreak, S s) {
  if (l(s) < charsPerLine) ret s;
  
  new StringBuilder buf;
  int n = l(s), j = 0;

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

Began life as a copy of #1033233