Return True if the input value is valid for insertion into the
        inner list.

        Args:
            value: An object about to be inserted.

def _is_valid(self, value):
        """Return True if the input value is valid for insertion into the
        inner list.

        Args:
            value: An object about to be inserted.
        """

        # Entities have an istypeof method that can perform more sophisticated
        # type checking.
        if hasattr(self._type, "istypeof"):
            return self._type.istypeof(value)
        else:
            return isinstance(value, self._type)