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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 _create_thumbnail(self, source_image, geometry_string, options, thumbnail):
 """ Creates the thumbnail by using default.engine """

        logger.debug('Creating thumbnail file [%s] at [%s] with [%s]',
                     thumbnail.name, geometry_string, options)
        ratio = default.engine.get_image_ratio(source_image, options)
        geometry = parse_geometry(geometry_string, ratio)
        image = default.engine.create(source_image, geometry, options)
        default.engine.write(image, options, thumbnail)
        # It's much cheaper to set the size here
        size = default.engine.get_image_size(image)
        thumbnail.set_size(size)