Catching a spy

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A spy is located on a one-dimensional line.  At time 0, the spy is at location A.  With each time interval, the spy moves B units to the right (if B is negative, the spy is moving left).  A and B are fixed integers, but they are unknown to you.  You are to catch the spy.  The means by which you can attempt to do that is:  at each time interval (starting at time 0), you can choose a location on the line and ask whether or not the spy is currently at that location.  That is, you will ask a question like “Is the spy currently at location 27?” and you will get a yes/no answer.  Devise an algorithm that will eventually find the spy.

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