code: func (ctx *ServiceContext) Service(service interface{}) error {
  	element := reflect.ValueOf(service).Elem()
  	if running, ok := ctx.services[element.Type()]; ok {
  		element.Set(reflect.ValueOf(running))
  		return nil
  	}
  	return ErrServiceUnknown
  }
docs: // Service retrieves a currently running service registered of a specific type.