code: func (s *Service) AddCharacteristic(c *Characteristic) *Characteristic {
  	for _, x := range s.Characteristics {
  		if x.UUID.Equal(c.UUID) {
  			panic("service already contains a characteristic with UUID " + c.UUID.String())
  		}
  	}
  	s.Characteristics = append(s.Characteristics, c)
  	return c
  }
docs: // AddCharacteristic adds a characteristic to a service.
  // AddCharacteristic panics if the service already contains another characteristic with the same UUID.