code: func (imf *ImmutableForest) newTree(prefix []byte) *RWTree {
  	p := string(prefix)
  	tree := NewRWTree(NewPrefixDB(imf.treeDB, p), imf.cacheSize)
  	imf.treeCache.Add(p, tree)
  	return tree
  }
docs: // Create a new in-memory IAVL tree