anchor.diff:  -spec label(cerl:cerl(), integer()) -> {cerl:cerl(), integer()}.
   
   label(T, N) ->
  -    label(T, N, dict:new()).
  +    label(T, N, #{}).
   
   label(T, N, Env) ->
       case type(T) of
  
positive.code:     {[], N}.
  
  label_vars([T | Ts], N, Env) ->
      Env1 = dict:store(var_name(T), N, Env),
      {As, N1} = label_ann(T, N),
      T1 = set_ann(T, As),
      {Ts1, N2, Env2} = label_vars(Ts, N1, Env1),
  
negative.code: 	    %% Constant literals are not labeled.
  	    {T, N};
  	var ->
              VarName = var_name(T),
              {As, N1} =
                  case Env of
                      #{VarName := L} ->
  		        {A, _} = label_ann(T, L),
  		        {A, N};
                      #{} ->
  		        label_ann(T, N)
                  end,
  	    {set_ann(T, As), N1};
  