docs: def _get_non_defined(self, data, class_types):
          """
          returns a list of URIs and blanknodes that are not defined within
          the dataset. For example: schema:Person has an associated rdf:type
          then it is considered defined.
  
          args:
              data: a list of triples
              class_types: list of subjects that are defined in the dataset
          """
          subj_set = set([item[self.smap] for item in class_types])
          non_def_set = set([item[self.smap] for item in data])
          return list(non_def_set - subj_set)