What do you think "Das Lila Lied" ("The Lilac Song") means to Bertie, Sofie, Karl, and Gert? What do you think it represents in the story itself?

Book Clubs The Lilac People by Milo Todd Discussion
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What do you think “Das Lila Lied” (“The Lilac Song”) means to Bertie, Sofie, Karl, and Gert? What do you think it represents in the story itself?