anchor.diff:                            end,
                             Replacements);
                   {maps, omitted} ->
  -                    ?expr(case maps:find('<FName>', 'M') of
  -                              error ->
  -                                  ok;
  -                              {ok, '<F>'} ->
  -                                  '<verify-fn>'('<F>', ['<FName>' | Path])
  +                    ?expr(case 'M' of
  +                              '#{<FName> := <F>}' ->
  +                                  '<verify-fn>'('<F>', ['<FName>' | Path]);
  +                              _ ->
  +                                  ok
                             end,
  -                          [replace_tree('M', MsgVar) | Replacements])
  +                          [replace_tree('#{<FName> := <F>}',
  +                                        map_match([{FName, FVar}])),
  +                           replace_tree('M', MsgVar) | Replacements])
               end
       end;
   field_verifier(#gpb_oneof{name=FName, fields=OFields}, FVar, MsgVar, Opts) ->
  
positive.code:                                    replace_term('merge_msg_X', MergeFn),
                                     replace_tree('New', NewValue)]);
                          pass_as_record ->
                              ?expr(case maps:find('fieldname', 'Msg') of
                                        error ->
                                            {'tag', 'New'};
                                        {ok, {'tag', MVPrev}} ->
                                            {'tag', 'merge_msg_X'(MVPrev, 'New')};
                                        _ ->
                                            {'tag', 'New'}
                                    end,
                                    [replace_term('fieldname', CFName),
                                     replace_term('tag', FName),
                                     replace_tree('Msg', MsgVar),
                                     replace_term('merge_msg_X', MergeFn),
  
negative.code:      replace_tree('NF', NF),
       replace_tree('Var', Var),
       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}]))].
  
  occurs_as_optional_submsg(MsgName, #anres{msg_occurrences=Occurrences}=AnRes) ->
      %% Note: order of evaluation below is important (the exprs of `andalso'):
  