Having served in World War I, German-born French painter and sculptor Max Ernst at first gravitated toward the Dada movement, but the former student of psychology and philosophy eventually became one of the founders of surrealism. Apart from the medium of collage, for which he is well known, Ernst developed other devices to express his fantastic vision, like frottage, in which a drawing tool is rubbed over paper laid on a textured surface, and grattage, a technique consisting of what? Discuss