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A heuristic

Explanation:

A heuristic is an approach to problem solving in which previous experiences are used to develop or derive a strategy to solve a problem. This method does not necessarily mean that it will be perfect for the goal or problem, but it is sufficient to provide a quick access (like a shortcuts) to the solution given the time frame. This is the reason why Joe asked the librarian, because this would save him a lot of time, and this would provide quick access to the book he is looking for. He then banks on the experience of the librarian to help him locate the book in a very short period of time instead of examining all books on the shelf.