code: private byte firstL_R_AL() {
          byte result = ON;
          for (int i = 0; i < prologue.length(); ) {
              int uchar = prologue.codePointAt(i);
              i += Character.charCount(uchar);
              byte dirProp = (byte)getCustomizedClass(uchar);
              if (result == ON) {
                  if (dirProp == L || dirProp == R || dirProp == AL) {
                      result = dirProp;
                  }
              } else {
                  if (dirProp == B) {
                      result = ON;
                  }
              }
          }
          return result;
      }
docs: Returns the directionality of the first strong character
  after the last B in prologue, if any.
  Requires prologue!=null.