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