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I think McEwan includes this allusion to show that Briony has grown as a writ and person in a very specific way. During all of Part I and the first half of Part III, most of her actions are in service of the perfect story, and the reason she gets Robbie in trouble is in service of a story as well. However, by including this allusion (more fully explained in the epilogue), McEwan shows that she has progressed beyond this unrealistic ideal.