anchor.diff:  is_reg_used_at_1(R, Lbl, St0) ->
       case check_liveness_at(R, Lbl, St0) of
   	{killed,St} -> {false,St};
  -	{used,St} -> {true,St};
  -	{unknown,St} -> {true,St}
  +	{used,St} -> {true,St}
       end.
   
   index_labels_1([{label,Lbl}|Is0], Acc) ->
  
positive.code:     St = #live{bl=fun check_used_block/3,lbl=D,res=gb_trees:empty()},
      case check_liveness_at(R, Lbl, St) of
  	{killed,_} -> true;
  	{used,_} -> false;
  	{unknown,_} -> false
      end.
  
  %% index_labels(FunctionIs) -> State
  
negative.code: 	{x,_} ->
  	    {killed,St};
  	_ ->
  	    %% y register. Rarely happens. Be very conversative and
  	    %% assume it's used.
  	    {used,St}
      end;
  check_liveness(R, [{loop_rec_end,{f,Fail}}|_], St) ->
      check_liveness_at(R, Fail, St);
  