anchor.diff:                                                  AccId,
                                                   BindFun,
                                                   ReadFun,
  -                                                undefined,
                                                   undefined]),
       ok.
   
   -spec internal_fold_harness(store(), id(), function(), id(), function(),
  -                            function(), value(), value()) -> function().
  +                            function(), value()) -> function().
   internal_fold_harness(Variables, Id, Function, AccId, BindFun, ReadFun,
  -                      PreviousValue, _PreviousAccValue) ->
  +                      PreviousValue) ->
       %% Blocking threshold read on source value.
       {ok, {Id, Type, Value, _}} = ReadFun(Id,
                                            {strict, PreviousValue},
  
positive.code: 
  %% Exported helper functions.
  -export([internal_fold/6,
           internal_fold_harness/8]).
  
  %% Exported utility functions.
  -export([next_key/3,
  
negative.code:     {ok, _} = BindFun(AccId, AccValue, Variables),
  
      internal_fold_harness(Variables, Id, Function, AccId, BindFun,
                            ReadFun, Value).
  
  %% @doc Send responses to waiting threads, via messages.
  %%
  