return nsPromiseSubstring<CharT>(aString, aStart, aEnd);
return nsPromiseSubstring<PRUnichar>(aStart, aEnd);
Substring( const basic_nsAReadableString<CharT>& aString, const nsReadingIterator<CharT>& aStart, const nsReadingIterator<CharT>& aEnd )  {    return nsPromiseSubstring<CharT>(aString, aStart, aEnd);  }