anchor.diff:  -type norm_opts() ::
       #{
           space => allowed | skip | integer(),
  -        % underscore => allowed | skip | integer(),
  -        unrecognized => skip | integer(),
  +        unrecognized => skip | keep | integer(),
           allowed => [integer()]
       }.
   
  
positive.code:     case maps:get(unrecognized, Opts, skip) of
          skip ->
              skip;
          Char when is_integer(Char) ->
              Char
      end.
  
  
  
  
negative.code: %% EUnit tests
  %% ===================================================================
  
  -define(TEST, true).
  -ifdef(TEST).
  -include_lib("eunit/include/eunit.hrl").
  
  