@classmethod
def from_image(klass, img):
    """ Class method to create new instance of own class from `img`

        Parameters
        ----------
        img : instance
            In fact, an object with the API of :class:`DataobjImage`.

        Returns
        -------
        cimg : instance
            Image, of our own class
        """
    if isinstance(img, klass):
        return img
    raise NotImplementedError
