anchor.diff:    {gpio_pin, Pin}.
   
   
  -%% @doc value_change(Old, New)
  +%% value_change(Old, New)
   %% value_change(0, 1) ->
   %%   raising;
   %% value_change(1, 0) ->
  
positive.code: %% process spawned.
  %% @todo Add init for exclusive pins
  %% @end
  -spec start_link(pin(), pin_direction()) ->
                      {'ok', pid()} | 'ignore' | {'error', term()}.
  start_link(Pin, Direction) ->
    case gproc:lookup_local_name(pname(Pin)) of
      undefined ->
        gen_server:start_link(?MODULE, [Pin, Direction], []);
  
negative.code: -behaviour(gen_server).
  
  %% API
  -export([start_link/1,
           release/1,
           write/2,
           read/1,
  