private String parseIdentifier() {
    StringBuilder sb = new StringBuilder();
    char c = next(false);
    if ( Character.isLetter(c) || c == '_' ) {
      sb.append(c);
      while ( hasNext() ) {
	c = next(false);
	if ( Character.isLetterOrDigit(c) || c == '_' ) {
	  sb.append(c);
	}
	else {
	  pushback();
	  break;
	}
      }
    }
    else {
      pushback();
    }
    return sb.toString();
  }