code: func (cs *clientStore) lookupByConnIDOrID(ID string, connID []byte) *client {
  	var c *client
  	if len(connID) > 0 {
  		c = cs.connIDs[string(connID)]
  	} else {
  		c = cs.clients[ID]
  	}
  	return c
  }
docs: // Lookup client by ConnID if not nil, otherwise by clientID.
  // Assume at least clientStore RLock is held on entry.