code: def get_payer_channel(
          channelidentifiers_to_channels: ChannelMap,
          transfer_pair: MediationPairState,
  ) -> Optional[NettingChannelState]:
      
      payer_channel_identifier = transfer_pair.payer_transfer.balance_proof.channel_identifier
      return channelidentifiers_to_channels.get(payer_channel_identifier)
docs: Returns the payer channel of a given transfer pair or None if it's not found