code: func (imf *ImmutableForest) tree(prefix []byte) (*RWTree, error) {
  	// Try cache
  	if value, ok := imf.treeCache.Get(string(prefix)); ok {
  		return value.(*RWTree), nil
  	}
  	// Not in caches but non-negative version - we should be able to load into memory
  	return imf.loadOrCreateTree(prefix)
  }
docs: // Lazy load tree