code: func (is *Isolators) Unrecognized() Isolators {
  	u := Isolators{}
  	for _, i := range *is {
  		if i.value == nil {
  			u = append(u, i)
  		}
  	}
  	return u
  }
docs: // Unrecognized returns a set of isolators that are not recognized.
  // An isolator is not recognized if it has not had an associated
  // constructor registered with AddIsolatorValueConstructor.