docs: def validate(self, value, model=None, context=None):
          """
          Validate
          Perform value validation against validation settings and return
          error object.
  
          :param value:           str, value to check
          :param model:           parent model being validated
          :param context:         object or None, validation context
          :return:                shiftschema.result.Error
          """
  
          if value not in self.choices:
              return Error(self.invalid_choice)
  
          # success otherwise
          return Error()