def marked_letters
      return if not_registered
      marked_choices.map do |q|
        q.map { |cho| (65+cho).chr }
      end
    end
def marked_letters
      return if not_registered
      marked_choices.map do |q|
        q.map { |cho| (65+cho).chr }
      end
    end
The set of letters marked on the response sheet. At this time, only the
 latin sequence 'A, B, C...' is supported.

 @return [Array] an array of arrays of 1-character strings; each element
   contains the list of letters marked for the corresponding question.