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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 _combine_transfers(self, result):
 """Combine multiple pair transfers into one."""

        transfers = {}
        for reaction_id, c1, c2, form in result:
            key = reaction_id, c1, c2
            combined_form = transfers.setdefault(key, Formula())
            transfers[key] = combined_form | form

        for (reaction_id, c1, c2), form in iteritems(transfers):
            yield reaction_id, c1, c2, form