anchor.diff:  %%%===================================================================
   
   init([DataRoot]) ->
  -    process_flag(trap_exit, true),
       schedule_tick(),
       Tree = hashtree_tree:new(cluster_meta, [{data_dir, DataRoot}, {num_levels, 2}]),
       State = #state{tree=Tree,
  
positive.code: 
  %% @private
  build_async(State) ->
      Pid = spawn_link(fun build/0),
      State#state{built=Pid}.
  
  %% @private
  build() ->
  
negative.code:           %% the tree managed by this process
            tree  :: hashtree_tree:tree(),
  
            %% whether or not the tree has been built or a monitor ref
            %% if the tree is being built
            built :: boolean() | reference(),
  
            %% a monitor reference for a process that currently holds a
            %% lock on the tree. undefined otherwise
  