Merge image channels.

        Parameters
        ----------
        imgs : `list` of `PIL.Image.Image`

        Returns
        -------
        `PIL.Image.Image`

        Raises
        ------
        ValueError
            If image channel list is empty.

def merge(self, imgs):
        """Merge image channels.

        Parameters
        ----------
        imgs : `list` of `PIL.Image.Image`

        Returns
        -------
        `PIL.Image.Image`

        Raises
        ------
        ValueError
            If image channel list is empty.
        """
        if not imgs:
            raise ValueError('empty channel list')
        if len(imgs) == 1:
            return imgs[0]
        return Image.merge(self.mode, imgs)