static Set<String> parseLetters(String line) {
        Matcher matcher = LETTER_PATTERN.matcher(line);
        Set<String> set = new LinkedHashSet<>();

        if (!matcher.find()) {
            return set;
        }

        String[] letters = matcher.group(2).split("[ \t]");

        for (String letter : letters) {
            String candidate = letter.trim();

            if (candidate.length() > 0) {
                set.add(candidate);
            }
        }

        return set;
    }