anchor.diff:      ReadRightFun = fun(_Right, _Threshold, _Variables) ->
               ?MODULE:read_var(_Right, _Threshold, _Variables)
       end,
  -    product(Left, Right, Product, Store, ?WRITE, ReadLeftFun, ReadRightFun).
  +    product(Left, Right, Product, Store, ?BACKEND_WRITE, ReadLeftFun, ReadRightFun).
   
   %% @doc Perform a read for a particular identifier.
   %%
  
positive.code:     ReadRightFun = fun(_Right, _Threshold, _Variables) ->
              ?MODULE:read_var(_Right, _Threshold, _Variables)
      end,
      intersection(Left, Right, Intersection, Store, ?WRITE, ReadLeftFun, ReadRightFun).
  
  %% @doc Compute the union 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.
  