Bibliographical noteFunding Information:
This work was carried out in the stimulating atmosphere of the Mittag– Leffler Institute. We thank F. Sottile and C. Greene for help with the literature on rim hooks, and C. Woodward and S. Agnihotri for the material in Section 5. The second named author was supported by a Mittag–Leffler Institute postdoctoral fellowship. The third author was supported by a Tage Erlander Guest Professorship and by the National Science Foundation.