code: func (m *MembershipStore) MsgByID(pkiID common.PKIidType) *proto.SignedGossipMessage {
  	m.RLock()
  	defer m.RUnlock()
  	if msg, exists := m.m[string(pkiID)]; exists {
  		return msg
  	}
  	return nil
  }
docs: // MsgByID returns a message stored by a certain ID, or nil
  // if such an ID isn't found