static S replaceNumbersWithVars(S s, final F0<S> varMaker) {
  final new SS map;
  ret regexpReplace(s, "\\d+", func(Matcher m) -> S {
    S number = m.group();
    S var = map.get(number);
    if (var == null)
      map.put(number, var = varMaker!);
    ret var;
  });
}