code: func (c *Client) checkCoordinate(coord *Coordinate) error {
  	if !c.coord.IsCompatibleWith(coord) {
  		return fmt.Errorf("dimensions aren't compatible")
  	}
  
  	if !coord.IsValid() {
  		return fmt.Errorf("coordinate is invalid")
  	}
  
  	return nil
  }
docs: // checkCoordinate returns an error if the coordinate isn't compatible with
  // this client, or if the coordinate itself isn't valid. This assumes the mutex
  // has been locked already.