code: def associations(self, subject, object=None):
          
          if object is None:
              if self.associations_by_subj is not None:
                  return self.associations_by_subj[subject]
              else:
                  return []
          else:
              if self.associations_by_subj_obj is not None:
                  return self.associations_by_subj_obj[(subject,object)]
              else:
                  return []
docs: Given a subject-object pair (e.g. gene id to ontology class id), return all association
          objects that match.