anchor.diff:  -spec fresh(vclock_node(), counter()) -> vclock().
   fresh(Node, Count) ->
       [{Node, {Count, timestamp()}}].
  -
  +%计算Va是否是Vb的后裔
   % @doc Return true if Va is a direct descendant of Vb, else false -- remember, a vclock is its own descendant!
   -spec descends(Va :: vclock(), Vb :: vclock()) -> boolean().
   descends(_, []) ->
  
positive.code:     true;
  descends(Va, Vb) ->
      [{NodeB, {CtrB, _T}}|RestB] = Vb,
      case lists:keyfind(NodeB, 1, Va) of
          false ->
              false;
          {_, {CtrA, _TSA}} ->
              (CtrA >= CtrB) andalso descends(Va,RestB)
          end.
  
  
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() ->
  