docs: def is_in(property_name, set_values, *, present_optional=False, message=None):
      """Returns a Validation that checks that a value is contained within a given set."""
      def check(val):
        """Checks that a value is contained within a scope-enclosed set."""
        if val is None:
          return present_optional
        else:
          return val in set_values
  
      return Validation(check, property_name, message)