anchor.diff:      rem_unused(Is, Used, []).
   
   rem_unused([{label,Lbl}=I|Is0], Used, [Prev|_]=Acc) ->
  -    case gb_sets:is_member(Lbl, Used) of
  +    case cerl_sets:is_element(Lbl, Used) of
   	false ->
   	    Is = case is_unreachable_after(Prev) of
   		     true -> drop_upto_label(Is0);
  
positive.code: 
  %% Add one or more label to the set of used labels.
  
  label_used({f,L}, St) -> St#st{labels=gb_sets:add(L, St#st.labels)};
  label_used([H|T], St0) -> label_used(T, label_used(H, St0));
  label_used([], St) -> St;
  label_used(_Other, St) -> St.
  
negative.code: %% Test if label is used.
  
  is_label_used(L, St) ->
      cerl_sets:is_element(L, St#st.labels).
  
  %% is_unreachable_after(Instruction) -> boolean()
  %%  Test whether the code after Instruction is unreachable.
  