code: def gen_references(self) -> str:
          
          rval = []
          for cls in self.schema.classes.values():
              pkeys = self.primary_keys_for(cls)
              for pk in pkeys:
                  pk_slot = self.schema.slots[pk]
                  classname = camelcase(cls.name) + camelcase(pk)
                  if cls.is_a and getattr(self.schema.classes[cls.is_a], pk, None):
                      parent = self.range_type_name(pk_slot, cls.is_a)
                  else:
                      parent = self.python_name_for(pk_slot.range)
                  rval.append(f'class {classname}({parent}):\n\tpass')
          return '\n\n\n'.join(rval)
docs: Generate python type declarations for all identifiers (primary keys)