code: def validate(self, value, model=None, context=None):
          
          invalid = [item for item in value if item not in self.choices]
          if len(invalid):
              return Error(
                  self.invalid_multichoice,
                  dict(items=', '.join(invalid))
              )
  
          # success otherwise
          return Error()
docs: Validate
          Perform value validation against validation settings and return
          error object.
  
          :param value:           list, value to check
          :param model:           parent model being validated
          :param context:         object or None, validation context
          :return:                shiftschema.result.Error