anchor.diff:  %% #"" regex
   %%------------------------------------------------------------------------------
   
  +-spec read_regex(state()) -> state().
   read_regex(#{src := <<>>}) ->
     throw(<<"EOF">>);
   read_regex(#{src := <<"\\"/utf8, Ch/utf8, _/binary>>} = State) ->
  
positive.code: %% Unquote
  %%------------------------------------------------------------------------------
  
  read_unquote(#{src := <<"~"/utf8, Src/binary>>} = State) ->
    case Src of
      <<"@", _/binary>> ->
  
negative.code: %% Symbol
  %%------------------------------------------------------------------------------
  
  -spec read_symbol(state()) -> state().
  read_symbol(State) ->
    Loc  = maps:get(loc, State),
    Meta = #{loc => Loc},
  