anchor.diff:      ct:pal("Parse Tree: ~p", [ParseTree]),
   
       %% Create and return identifier.
  -    OutputId = materialize(ParseTree, #state{set_id=?DEFAULT}),
  +    OutputId = materialize(ParseTree, #state{output_id=OutputId}),
   
       %% Return output identifier.
       {ok, OutputId}.
  
positive.code: 
  extract(Variable, Tuple) ->
      maps:get(Variable, Tuple).
  
negative.code: 
  %% TODO: Single projection only!
  materialize({select, Projections},
              #state{output_id=OutputId, predicate_output_id=PredicateOutputId}=State) ->
  
      %% Apply the projection.
      lasp:map(PredicateOutputId,
  