anchor.diff:  %%
   -spec bind_to(id(), id(), store(), function()) -> {ok, pid()}.
   bind_to(AccId, Id, Store, BindFun) ->
  -    bind_to(AccId, Id, Store, BindFun, ?READ).
  +    bind_to(AccId, Id, Store, BindFun, ?BACKEND_READ).
   
   bind_to(AccId, Id, Store, BindFun, ReadFun) ->
       TransFun = fun({_, _, _, V}) -> V end,
  
positive.code:     ReadRightFun = fun(_Right, _Threshold, _Variables) ->
              ?MODULE:read_var(_Right, _Threshold, _Variables)
      end,
      union(Left, Right, Union, Store, ?WRITE, ReadLeftFun, ReadRightFun).
  
  %% @doc Compute the cartesian product of two sets.
  %%
  
negative.code: %%      stream can result in observable nondeterminism.
  %%
  stream(Id, Function, Store) ->
      stream(Id, Function, Store, ?BACKEND_READ).
  
  %% @doc Stream values out of the Lasp system; using the values from this
  %%      stream can result in observable nondeterminism.
  