anchor.diff:  
   filter_ops(Ops) ->
       %% TODO: Filter out only downstream update operations from log
  -    [ Op || { _Key, Op } <- Ops ].
  +    [Op || { _Key, Op} <- Ops].
  
positive.code:     riak_core_vnode_master:get_vnode_pid(I, ?MODULE).
  
  %% @doc Read state of key at given snapshot time
  -spec read(Key::term(), Type::term(), Snapshot_time::vectorclock:vectorclock())
            -> {ok, term()} | {error, term()}.
  read(Key, Type, Snapshot_time) ->
      DocIdx = riak_core_util:chash_key({?BUCKET,
                                         term_to_binary(Key)}),
  
negative.code:       NewPref, {read, Key, Type, Snapshot_time}, materializer_vnode_master).
  
  %%@doc write downstream operation to persistant log and cache it for future read
  -spec update(key(), #clocksi_payload{}) -> ok | {error, atom()}.
  update(Key, DownstreamOp) ->
      DocIdx = riak_core_util:chash_key({?BUCKET, term_to_binary(Key)}),
      Preflist = riak_core_apl:get_primary_apl(DocIdx, 1, materializer),
      [{NewPref,_}] = Preflist,
      riak_core_vnode_master:sync_command(NewPref, {update, Key, DownstreamOp},
  