function param($param, $type = null) {
		if ($param instanceof ExpressibleInterface) {
			$param = $param->get();
		}
		if ($param instanceof Expr) {
			return (string) $param;
		}
		
		$this->params[] = $param;
		$this->types[]  = $type;
		
		return "\$".count($this->params);
	}