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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _prepare_facet_field_spies(self, facets):
 """ Returns a list of spies based on the facets used to count frequencies. """

        spies = []
        for facet in facets:
            slot = self.column[facet]
            spy = xapian.ValueCountMatchSpy(slot)
            # add attribute "slot" to know which column this spy is targeting.
            spy.slot = slot
            spies.append(spy)
        return spies