code: func NewMemoryIndex() *Index {
  	ix, err := New(sorted.NewMemoryKeyValue())
  	if err != nil {
  		// Nothing to fail in memory, so worth panicing about
  		// if we ever see something.
  		panic(err)
  	}
  	return ix
  }
docs: // NewMemoryIndex returns an Index backed only by memory, for use in tests.