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