/**
     * Generate a poem from an input string.
     *
     * @param input string from which to create the poem
     * @return poem (as described above)
     */
    public String poem(String input) {
        List<String> output = new ArrayList<>();
        try (Scanner s = new Scanner(input).useDelimiter("\\s+")) {
            String old = "";
            while (s.hasNext()) {
                String cur = s.next();
                String prev = old.toLowerCase();
                String next = cur.toLowerCase();
                if (!old.isEmpty()) {
                    output.add(old);
                    Optional<String> bestBridge = graph.targets(prev).entrySet().stream()
                            .filter(e -> graph.targets(e.getKey()).containsKey(next))
                            .map(e -> e.getKey())
                            .collect(Collectors.toMap(e -> e,
                                                      e -> graph.sources(e).get(prev) + graph.targets(e).get(next)))
                            .entrySet().stream()
                            .sorted((a,b) -> b.getValue().compareTo(a.getValue()))
                            .findFirst().map(e -> e.getKey());
                    bestBridge.ifPresent(bridge -> output.add(bridge));
                }
                old = cur;
            }
            output.add(old);
        }
        checkRep();
        return output.stream().reduce((a,b) -> a + " " + b).orElse("");
    }