anchor.diff:           '<merge-it>',
            do_exprs(fun render_omissible_merger/2,
                     render_field_mergers(MsgName, MandatoryMergings, Opts),
  -                  OptMergings)),
  -       splice_clauses('<msg-undefined-handling>', MsgUndefFnClauses)]).
  -
  -compute_msg_merger_undef_clauses(MsgName, AnRes, Opts) ->
  -    case occurs_as_optional_submsg(MsgName, AnRes) of
  -        false ->
  -            %% If it cannot occur as optional sub message,
  -            %% then it cannot be called with either Prev or New being
  -            %% undefined. Don't layout code for it. Eg. Dialyzer will
  -            %% find it and complain.
  -            [];
  -        true ->
  -            case get_mapping_and_unset_by_opts(Opts) of
  -                Y when Y == records;
  -                       Y == {maps, present_undefined} ->
  -                    [?fn_clause(fun(Prev, undefined) -> Prev end),
  -                     ?fn_clause(fun(undefined, New)  -> New end)];
  -                {maps, omitted} ->
  -                    %% Similar to above: The entire map is always
  -                    %% matched to a variable.
  -                    %% FIXME: also if all field are non-optionals??
  -                    []
  -            end
  -    end.
  +                  OptMergings))]).
   
   format_msg_merger_fnclause_match(_MsgName, [], _Opts) ->
       {?expr(PF), ?expr(_), no_fields};
  
positive.code: splice_trees(Marker, Trees) when is_atom(Marker) ->
      {splice_trees, Marker, Trees}.
  
  splice_clauses(Marker, Clauses) when is_atom(Marker) ->
      {splice_clauses, Marker, Clauses}.
  
  repeat_clauses(Marker, RepetitionReplacements) ->
      {repeat_clauses, Marker, RepetitionReplacements}.
  
  
negative.code:      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),
  