code: def by_type(self, identifier_class):
          
          myidentifiers = set()
          for identifier in self.source_identifiers.values():
              if (isinstance(identifier, identifier_class)):
                  myidentifiers.update([identifier])
          return set(myidentifiers)
docs: returns all unique instances of a type of identifier
  
          :param identifier_class: the class to match identifier with
          :returns: a tuple