code: func (whisper *Whisper) RequestHistoricMessages(peerID []byte, envelope *Envelope) error {
  	p, err := whisper.getPeer(peerID)
  	if err != nil {
  		return err
  	}
  	p.trusted = true
  	return p2p.Send(p.ws, p2pRequestCode, envelope)
  }
docs: // RequestHistoricMessages sends a message with p2pRequestCode to a specific peer,
  // which is known to implement MailServer interface, and is supposed to process this
  // request and respond with a number of peer-to-peer messages (possibly expired),
  // which are not supposed to be forwarded any further.
  // The whisper protocol is agnostic of the format and contents of envelope.