code: private byte lastL_R_AL() {
          for (int i = prologue.length(); i > 0; ) {
              int uchar = prologue.codePointBefore(i);
              i -= Character.charCount(uchar);
              byte dirProp = (byte)getCustomizedClass(uchar);
              if (dirProp == L) {
                  return _L;
              }
              if (dirProp == R || dirProp == AL) {
                  return _R;
              }
              if(dirProp == B) {
                  return _ON;
              }
          }
          return _ON;
      }
docs: Returns the directionality of the last strong character at the end of the prologue, if any.
  Requires prologue!=null.