anchor.diff:      beam_ssa:fold_instrs_rpo(F, [0], no, Blocks).
   
   opt_ref_used(RecvLbl, Ref, Blocks) ->
  -    Vs = #{{var,Ref}=>ref,ref=>Ref,ref_matched=>false},
  +    Vs = #{Ref=>ref,ref=>Ref,ref_matched=>false},
       case opt_ref_used_1(RecvLbl, Vs, Blocks) of
           used -> true;
           not_used -> false;
  
positive.code: 
  update_vars(#b_set{args=Args,dst=Dst}, Vs) ->
      Vars = [V || #b_var{}=V <- Args],
      All = all(fun(V) ->
                        Var = {var,V},
                        case Vs of
                            #{Var:=message} -> true;
                            #{} -> false
                        end
                end, Vars),
      case All of
          true -> Vs#{{var,Dst}=>message};
          false -> Vs
      end.
  
  
negative.code:     end.
  
  opt_ref_used_is([#b_set{op=peek_message,dst=Msg}|Is], Vs0) ->
      Vs = Vs0#{Msg=>message},
      opt_ref_used_is(Is, Vs);
  opt_ref_used_is([#b_set{op={bif,Bif},args=Args,dst=Dst}=I|Is],
                  Vs0) ->
  