code: func IncludeUnset(b bool) source.Option {
  	return func(o *source.Options) {
  		if o.Context == nil {
  			o.Context = context.Background()
  		}
  		o.Context = context.WithValue(o.Context, includeUnsetKey{}, true)
  	}
  }
docs: // IncludeUnset toggles the loading of unset flags and their respective default values.
  // Default behavior is to ignore any unset flags.