function __construct( $xls_fname = null ) {
parent::__construct( $xls_fname );
# Create question writers; Single instances will be re-used in the top loop.
# They are sharing the same instance of Spreadsheet_Excel_Writer_Worksheet.
# We do not use class with static properties and methods only because
# such class names cannot be accessed by reference in PHP < 5.3.0:
# http://php.net/manual/en/language.oop5.static.php
# "As of PHP 5.3.0, it's possible to reference the class using a variable.
# The variable's value can not be a keyword (e.g. self, parent and static)."
$this->tabular_writer = new qp_XlsTabularQuestion();
$this->text_writer = new qp_XlsTextQuestion();
}
