When the book is used, it will be regretted. It will not be difficult until it has happened.

It will be too late to regret when the book is used, which means that you must have sufficient knowledge reserves, otherwise, you will regret it when you use it. Opportunity always favors those who are prepared. When opportunities come, only those who are prepared will seize them in time and take advantage of them to achieve success. Some things can't be done, and some things can't be done.