public function parse(Parser $parser)
{
$parser->match(Lexer::T_IDENTIFIER);
$parser->match(Lexer::T_OPEN_PARENTHESIS);
$this->expr1 = $parser->StringPrimary();
$parser->match(Lexer::T_COMMA);
$this->expr2 = $parser->StringPrimary();
$parser->match(Lexer::T_CLOSE_PARENTHESIS);
}