Combine multiple pair transfers into one.

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