code: func (p *peer) MarkTransaction(hash common.Hash) {
  	// If we reached the memory allowance, drop a previously known transaction hash
  	for p.knownTxs.Cardinality() >= maxKnownTxs {
  		p.knownTxs.Pop()
  	}
  	p.knownTxs.Add(hash)
  }
docs: // MarkTransaction marks a transaction as known for the peer, ensuring that it
  // will never be propagated to this particular peer.