code: def create_relationship(self, relationship_form=None):
          
          if relationship_form is None:
              raise NullArgument()
          if not isinstance(relationship_form, abc_relationship_objects.RelationshipForm):
              raise InvalidArgument('argument type is not a RelationshipForm')
          if relationship_form.is_for_update():
              raise InvalidArgument('form is for update only, not create')
          try:
              if self._forms[relationship_form.get_id().get_identifier()] == CREATED:
                  raise IllegalState('form already used in a create transaction')
          except KeyError:
              raise Unsupported('form did not originate from this session')
          if not relationship_form.is_valid():
              raise InvalidArgument('one or more of the form elements is invalid')
  
          url_path = ('/handcar/services/relationship/families/' +
                      self._catalog_idstr + '/relationships')
          try:
              result = self._post_request(url_path, relationship_form._my_map)
          except Exception:
              raise  # OperationFailed
          self._forms[relationship_form.get_id().get_identifier()] = CREATED
          return objects.Relationship(result)
docs: Creates a new ``Relationship``.
  
          arg:    relationship_form (osid.relationship.RelationshipForm):
                  the form for this ``Relationship``
          return: (osid.relationship.Relationship) - the new
                  ``Relationship``
          raise:  IllegalState - ``relationship_form`` already used in a
                  create transaction
          raise:  InvalidArgument - one or more of the form elements is
                  invalid
          raise:  NullArgument - ``relationship_form`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          raise:  Unsupported - ``relationship_form`` did not originate
                  from ``get_relationship_form_for_create()``
          *compliance: mandatory -- This method must be implemented.*