anchor.diff:  %% The name is general so declare local type for readability.
   -type index_result_keys() :: keys().
   
  +-define(DEFAULT_GC_BATCH_SIZE, 1000).
  +-define(DEFAULT_GC_WORKERS, 2).
  +
   -record(gc_batch_state, {
             %% start of the current gc batch
             batch_start :: undefined | non_neg_integer(),
  
positive.code:           %% start of the current gc interval
            start_key :: binary(),
            end_key :: binary(),
            batch_size=1 :: pos_integer(),
            %% Used for paginated 2I querying of GC bucket
            continuation :: continuation()
           }).
  
negative.code:           block_count=0 :: non_neg_integer(),
            leeway :: non_neg_integer(),
            worker_pids=[] :: [pid()],
            max_workers=?DEFAULT_GC_WORKERS :: pos_integer(),
            batch_size=?DEFAULT_GC_BATCH_SIZE :: pos_integer(),
            %% Used for paginated 2I querying of GC bucket
            key_list_state :: undefined | gc_key_list_state(),
            %% Options to use when start workers
  