code: func (muf *MutableForest) Writer(prefix []byte) (*RWTree, error) {
  	// Try dirty cache first (if tree is new it may only be in this location)
  	prefixString := string(prefix)
  	if tree, ok := muf.dirty[prefixString]; ok {
  		return tree, nil
  	}
  	tree, err := muf.tree(prefix)
  	if err != nil {
  		return nil, err
  	}
  	// Mark tree as dirty
  	muf.dirty[prefixString] = tree
  	muf.dirtyPrefixes = append(muf.dirtyPrefixes, prefixString)
  	return tree, nil
  }
docs: // Calls to writer should be serialised as should writes to the tree