code: func (d *Peer) seen(p *BzzAddr) bool {
  	d.mtx.Lock()
  	defer d.mtx.Unlock()
  	k := string(p.Address())
  	if d.peers[k] {
  		return true
  	}
  	d.peers[k] = true
  	return false
  }
docs: // seen takes a peer address and checks if it was sent to a peer already
  // if not, marks the peer as sent