I'm playing a concert tonight with a really wonderful program: Dohnanyi's "Variations on a Nursery Song," Britten's "Young Person's Guide to the Orchestra," and Elgar's "Enigma Variations." The Britten is, of course, not unrelated to the Elgar, but what I found going through my head between dreams last night was the concurrent juxtaposition of the theme that Dohnanyi uses and the variations of the Elgar. And then I realized that the "mystery" theme (one that everybody knows, yet nobody knows because it is absent) is the theme of the Dohnanyi. Pay special attention at the 3-minute mark of this video, and then consider the first Elgar Variation as a variation of this theme in the minor mode. The harmony is Elgar harmony, of course, and the relationship is obtuse, as the best variations on themes tend to be.