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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _channel(self, width, height, state_sizes, start_level, start_image, dataset):
 """Generate a channel. Parameters width : `int` Image width. height : `int` Image height. state_sizes : `list` of (`int` or `None`) Level state sizes. start_level : `int` Initial level. start_image : `PIL.Image` or `None` Initial level image. dataset : `str` Dataset key prefix. Returns ------- `PIL.Image` Generated image. """

        ret = start_image
        for level, state_size in enumerate(state_sizes, start_level + 1):
            key = dataset + level_dataset(level)
            if start_image is not None:
                scale = self.scanner.level_scale[level - 1]
                width *= scale
                height *= scale
            ret = self.imgtype.create_channel(width, height)
            if start_image is None:
                tr = self.scanner.traversal[0](width, height, ends=False)
                data = self._imgdata(width, height, state_size, '', key)
                self._write_imgdata(ret, data, tr)
            else:
                tr = self.scanner.traversal[0](
                    start_image.size[0],
                    start_image.size[1],
                    ends=False
                )
                for xy in tr:
                    start = pixel_to_state(start_image.getpixel(xy))
                    data = self._imgdata(scale, scale, state_size, start, key)
                    blk = self.scanner.traversal[level](scale, scale, False)
                    x, y = xy
                    x *= scale
                    y *= scale
                    self._write_imgdata(ret, data, blk, x, y)
            start_image = ret
        return ret