anchor.diff:      true;
   descends(Va, Vb) ->
       [{NodeB, {CtrB, _T}}|RestB] = Vb,
  +    %在Va中寻找Vb的最后一个节点
       case lists:keyfind(NodeB, 1, Va) of
           false ->
               false;
           {_, {CtrA, _TSA}} ->
  +        %Va的counter必须要大于等于Vb的counter
  +        %同时Va也必须是Vb剩余节点的后裔节点
               (CtrA >= CtrB) andalso descends(Va,RestB)
           end.
   
  
positive.code: -endif.
  
  -export_type([vclock/0, timestamp/0, vclock_node/0, dot/0, pure_dot/0]).
  
  -type vclock() :: [dot()].
  -type dot() :: {vclock_node(), {counter(), timestamp()}}.
  -type pure_dot() :: {vclock_node(), counter()}.
  
negative.code: -type   vclock_node() :: term().
  -type   counter() :: integer().
  -type   timestamp() :: integer().
  %创建一个空List
  % @doc Create a brand new vclock.
  -spec fresh() -> vclock().
  fresh() ->
  