code: def primary_keys_for(self, cls: ClassDefinition) -> List[SlotDefinitionName]:
          
          return [slot_name for slot_name in self.all_slots_for(cls)
                  if self.schema.slots[slot_name].primary_key or self.schema.slots[slot_name].identifier]
docs: Return all primary keys / identifiers for cls
  
          @param cls: class to get keys for
          @return: List of primary keys