anchor.diff:  %% when a block is written
   %% and we were already not full,
   %% we're still not full
  -not_full({block_written, _BlockID}, State) ->
  +not_full({block_written, _BlockID, _WriterPid}, State) ->
       %% 1. send a message to the
       %%    gen_server that sent us this
       %%    message to start writing
  
positive.code:     %%    unacked_writes set
      {next_state, not_full, State}.
  
  all_received({block_written, BlockID}, State=#state{unacked_writes=UnackedWrites}) ->
      %% 1. send a message to the
      %%    gen_server that sent us this
      %%    message to start writing
  
negative.code:     gen_fsm:sync_send_event(Pid, finalize).
  
  block_written(Pid, BlockID) ->
      gen_fsm:sync_send_event(Pid, {block_written, BlockID, self()}).
  
  %%%===================================================================
  %%% gen_fsm callbacks
  