“Practically speaking, it doesn’t matter where the Sendak materials live or who owns them. Any exhibition uses only a few dozen items at a time, and loans are common in the world of arts and literature. In theory, if the Rosenbach and the Sendak trustees agreed, a steady stream of Sendak shows could continue to flow through the Rosenbach, and as far as the backstage legal status and residency of the collection goes, the public would be none the wiser.”