anchor.diff:  -module(erlydtl_dateformat).
  --export([format/1, format/2, weeknum_year/3, year_weeknum/3]).
  +-export([format/1, format/2]).
   
   -define(TAG_SUPPORTED(C),
       C =:= $a orelse
  
positive.code:     C =:= $Y orelse
      C =:= $z orelse
      C =:= $Z orelse
      C =:= $o orelse
      C =:= $e
  ).
  
  %
  
negative.code: % always positive.
  tag_to_value($Z, _, _) ->
     "TODO";
  
  %% o – the ISO 8601 year number
  tag_to_value($o, {Y,M,D}, _) ->
  