anchor.diff:  %% API Function Exports
   %% ------------------------------------------------------------------
   
  --export([start_link/1]).
  +-export([start_link/1, stop/1]).
   
   %% send payloads
   -export([ping/1, order/6, order_cancel/2]).
  +-export([subscribe/2, unsubscribe/2]).
   
   %% ------------------------------------------------------------------
   %% gen_server Function Exports
  
positive.code:   {Reply, State} = send_call(Payload, State0),
    {reply, Reply, State}.
  
  handle_cast(_Msg, State) ->
    {noreply, State}.
  
  
negative.code: 
  start_link(Opts) ->
    gen_server:start_link(?MODULE, Opts, []).
  stop(Pid) ->
    gen_server:cast(Pid, stop).
  
  ping(Pid) ->
    gen_server:call(Pid, ping).
  