-- See *Gerth Stølting Brodal, Erik D. Demaine, Jeremy T. Fineman,
   John Iacono, Stefan Langerman, and J. Ian Munro, “Cache-Oblivious
   Dynamic Dictionaries with Optimal Update/Query Tradeoff", SODA
   2010* for a data structure which improves upon the shuttle
   tree. (2/18/2010)