return nsPromiseSubstring<CharT>(aString, aStartPos, aSubstringLength);
return nsPromiseSubstring<Chart>(aStart, aEnd);
Substring( const basic_nsAReadableString<CharT>& aString, PRUint32 aStartPos, PRUint32 aSubstringLength )  {    return nsPromiseSubstring<CharT>(aString, aStartPos, aSubstringLength);  }