[I got this problem from Radu Grigore.]

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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