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