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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:
 async def find(self, seq_set: SequenceSet, selected: SelectedMailbox, requirement: FetchRequirement = FetchRequirement.METADATA) \ -> AsyncIterable[Tuple[int, MessageT]]: """Find the active message UID and message pairs in the mailbox that are contained in the given sequences set. Message sequence numbers are resolved by the selected mailbox session. Args: seq_set: The sequence set of the desired messages. selected: The selected mailbox session. requirement: The data required from each message. """

        for seq, cached_msg in selected.messages.get_all(seq_set):
            msg = await self.get(cached_msg.uid, cached_msg, requirement)
            if msg is not None:
                yield (seq, msg)