anchor.diff:                        ++ OneofTransforms,
                     MmO = map_match([{FName, ?expr({'tag', 'OPF'}, Trs2)}]),
                     MmN = map_match([{FName, ?expr({'tag', 'ONF'}, Trs2)}]),
  -                  MergeCall = ?expr({'tag','merge'('OPF','ONF')}, Trs2),
  +                  MergeCall = ?expr({'tag','merge'('OPF','ONF', 'TrUserData')},
  +                                    Trs2),
                     [replace_tree(
                        '{#{fname := {tag,OPF}}, #{fname := {tag,ONF}}}',
                        ?expr({'#{fname := {tag,OPF}}', '#{fname := {tag,ONF}}'},
  
positive.code:      replace_tree('Var#{fname=>NF}', map_set(Var, [{FName, NF}])),
       replace_tree('Var#{fname=>PF}', map_set(Var, [{FName, PF}])),
       replace_tree('#{fname := NF}', map_match([{FName, NF}])),
       replace_tree('#{fname := PF}', map_match([{FName, PF}]))].
  
  format_field_skippers(MsgName, AnRes) ->
      SkipVarintFnName = mk_fn(skip_varint_, MsgName),
  
negative.code:             ["-spec merge_msgs(_, _) -> no_return().\n",
               gpb_codegen:format_fn(
                 merge_msgs,
                 fun(Prev, New) ->
                         merge_msgs(Prev, New, [])
                 end),
               "-spec merge_msgs(_, _, _) -> no_return().\n",
               gpb_codegen:format_fn(
                 merge_msgs,
                 fun(_Prev, _New, _Opts) ->
                         erlang:error({gpb_error, no_messages})
                 end)];
          maps ->
              ["-spec merge_msgs(_, _, _) -> no_return().\n",
               gpb_codegen:format_fn(
                 merge_msgs,
                 fun(Prev, New, MsgName) ->
                         merge_msgs(Prev, New, MsgName, [])
                 end),
               "-spec merge_msgs(_, _, _, _) -> no_return().\n",
               gpb_codegen:format_fn(
                 merge_msgs,
                 fun(_Prev, _New, _MsgName, _Opts) ->
                         erlang:error({gpb_error, no_messages})
                 end)]
      end.
  