anchor.diff:  -spec optimistic_slots() -> pos_integer().
   optimistic_slots() -> call(min_upload).
   
  --spec max_upload_rate() -> pos_integer().
  +-spec max_upload_rate() -> pos_integer() | infinity.
   max_upload_rate() -> call(max_upload_rate).
  +
  +-spec max_download_rate() -> pos_integer() | infinity.
  +max_download_rate() -> call(max_download_rate).
   
   -spec listen_port() -> pos_integer().
   listen_port() -> call(port).
  
positive.code:      optional(fs_watermark_high, 128),
       optional(max_upload_slots, auto),
       required(min_upload),
       required(max_upload_rate),
       required(port),
       required(logger_dir),
       required(logger_fname),
  
negative.code: 	 max_files/0,
  	 max_peers/0,
  	 max_upload_rate/0,
  	 max_download_rate/0,
  	 max_upload_slots/0,
  	 optimistic_slots/0,
  	 profiling/0,
  