anchor.diff:  start_device( Server, Configuration ) ->
   	gen_server:call( Server, { start_device, Configuration } ).
   
  --spec stop_device( pid(), device_id() ) -> ok.
  +-spec stop_device( pid(), device_id() ) -> ok | { error | no_such_device }.
   stop_device( Server, Id ) ->
   	gen_server:call( Server, { stop_device, Id } ).
   
  
positive.code: 
  
  handle_call( { stop_device, DeviceId }, _From, State ) ->
  	do_stop_device( DeviceId ),
  	NewDevices = orddict:erase( DeviceId, State#state.devices ),
  	NewState = State#state{ devices = NewDevices },
  	{ reply, ok, NewState };
  
  
  handle_call( list_devices, _From, State ) ->
  
negative.code: 	meck:new( lurch_driver_port, [ ] ),
  	meck:expect( lurch_driver_port, start_driver,
  				 fun( _Driver, _Parameters ) -> { ok, port_mock } end ),
  	meck:expect( lurch_driver_port, stop_driver,
  				 fun( Port ) -> ok end ),
  	Pid.
  
  
  