// returns (live value, updater)
static Pair<SimpleLiveValue<S>, Runnable> trimmedFileLiveValue(final File f, fS defaultValue) {
  final SimpleLiveValue<S> lv = stringLiveValue(trim(loadTextFile(f, defaultValue)));
  lv.onChange(r { saveTextFileIfDifferent(f, lv!) });
  ret pair(lv, (Runnable) r { lv.set(trim(loadTextFile(f, defaultValue))) });
}