public function stmt_fragment($part, $args=[]) {
		$type = $this->dbtype;
		if ($part == 'engine') {
			if ($type == 'mysql')
				# we only intent to support FOREIGN KEY-capable engines
				return "ENGINE=InnoDB";
			return '';
		}
		if ($part == "index") {
			if ($type == 'pgsql')
				return 'SERIAL PRIMARY KEY';
			if ($type == 'mysql')
				return 'INTEGER PRIMARY KEY AUTO_INCREMENT';
			return 'INTEGER PRIMARY KEY AUTOINCREMENT';
		}
		if ($part == 'datetime') {
			$delta = 0;
			if ($args && isset($args['delta']))
				$delta = (int)$args['delta'];
			return $this->stmt_fragment_datetime($delta);
		}
		return "";
	}