static S replaceRangesWithXYZVars(S s, L<IntRange> ranges) {
  new StringBuilder buf;
  int i = 0, varIndex = 0;
  for (IntRange r : ranges) {
    if (i < r.start) buf.append(substring(s, i, r.start));
    buf.append(xyzVar(++varIndex));
    i = r.end;
  }
  if (i < l(s)) buf.append(substring(s, i));
  ret str(buf);
}