function Spreadsheet_Excel_Writer_Parser($byte_order, $biff_version)
    {
        $this->_current_char  = 0;
        $this->_BIFF_version  = $biff_version;
        $this->_current_token = '';       // The token we are working on.
        $this->_formula       = '';       // The formula to parse.
        $this->_lookahead     = '';       // The character ahead of the current char.
        $this->_parse_tree    = '';       // The parse tree to be generated.
        $this->_initializeHashes();      // Initialize the hashes: ptg's and function's ptg's
        $this->_byte_order = $byte_order; // Little Endian or Big Endian
        $this->_ext_sheets = array();
        $this->_references = array();
    }