// a must be trimmed
sS replaceLine_keepingPadding(S text, S a, S b) {
  LS lines = lines(text);
  bool change = false;
  for i over lines: {
    S s = lines.get(i);
    S trimmed = trim(s);
    if (eqic(trimmed, a)) {
      int idx = indexOf(s, trimmed);
      lines.set(i, replaceSubstring(s, idx, idx+l(trimmed), b));
      set change;
    }
  }
  ret change ? lines_possiblyRTrim(text, lines) : text;
}