// no guarantees are made if you supply overlapping/out-of-bounds ranges
sS replaceCharRanges(S text, LPair<IntRange, S> replacements) {
  if (empty(replacements)) ret text;
  replacements = sortByPairsA(replacements);
  int iChar = 0, i = 0;
  new StringBuilder buf;
  while (i < l(replacements)) {
    Pair<IntRange, S> p = replacements.get(i++);
    buf.append(substring(text, iChar, p.a.start));
    buf.append(p.b);
    iChar = p.a.end;
  }
  if (iChar < l(text))
    buf.append(substring(text, iChar));
  ret str(buf);
}