Replaces all question mark ('?') occurrences with a random letter.

        :param text: string to be parsed
        :param letters: a set of letters to choose from.
        :returns: string with all letter placeholders filled in

def lexify(self, text='????', letters=string.ascii_letters):
        """
        Replaces all question mark ('?') occurrences with a random letter.

        :param text: string to be parsed
        :param letters: a set of letters to choose from.
        :returns: string with all letter placeholders filled in
        """
        return _re_qm.sub(lambda x: self.random_element(letters), text)