anchor.diff:       {history, {call,Backend,delete,[S#qcst.s,bkey()]}},
        {history, {call,Backend,list,[S#qcst.s]}},
        {history, {call,?MODULE,fold,[Backend,S#qcst.s]}},
  +     {{stopped, Backend, Volatile}, {call,Backend,drop,[S#qcst.s]}},
        {{stopped, Backend, Volatile}, {call,Backend,stop,[S#qcst.s]}}
       ].
   
  
positive.code:         false ->
              S1
      end;
  next_state_data(_From,_To,S,BeState,{call,_M,init_backend,_}) ->
      S#qcst{s = BeState};
  next_state_data(_From,_To,S,_R,{call,_M,put,[_S, Key, Val]}) ->
  
negative.code: initial_state_data(Config) ->
      #qcst{c = Config, d = orddict:new()}.
  
  next_state_data({running,Backend,Volatile},{stopped,Backend,Volatile},S,_R,
                  {call,_M,stop,_}) ->
      S1 = S#qcst{s = undefined, olds = sets:add_element(S#qcst.s, S#qcst.olds)},
      case Volatile of
          true ->
  