anchor.diff:      Pid ! { stop, From },
       wait( To ).
   
  --spec stop_async( pid() ) -> { ok, { pid(), reference() } } | { error, timeout }.
  +-spec stop_async( pid() ) -> { ok, msg_token() }.
   stop_async( Pid ) ->
       { From, To } = from_to( self(), Pid ),
       Pid ! { stop, From },
  
positive.code: 
  -spec enter_loop( string() | binary(),
                    [ string() | binary() ],
                    { pid(), reference() } ) ->
      { ok, { pid(), reference() } } | { error, term() }.
  enter_loop( Driver, Parameters, { Pid, Tag } ) ->
      case start_driver( Driver, Parameters ) of
          { ok, Port } ->
  
negative.code: from( ) ->
      { self(), make_ref() }.
  
  -spec wait( msg_token() ) ->
      any() | { error, timeout }.
  wait( Id ) ->
      receive
  