use Clair::SentenceFeatures qw(length_feature);
    use Clair::Document;
    my $doc = Clair::Document->new( ... );
    $doc->compute_sentence_feature(name => "length", 
        length_feature => \&length_feature );

    my %params = @_;
    my $state = $params{state};

    # Pre-compute the centroid
    unless (defined $state->{initialized}) {

        my $text;
        if (defined $params{cluster}) {
            $text = $params{cluster}->get_text();
        } else {
            $text = $params{document}->get_text();
        }

        open_nidf("$MEAD_HOME/etc/enidf");
        my $centroid = Clair::Centroid->new();
        $centroid->add_document($text);
        $state->{centroid} = $centroid;

        $state->{initialized} = 1;
    }

    my $sent = $params{sentence};
    return $state->{centroid}->centroid_score($sent);

    my %params = @_;
    my $sent_doc = Clair::Document->new(string => $params{sentence});
    my @words = $sent_doc->split_into_words();
    return scalar @words;

    my %params = @_;
    my $total = scalar $params{document}->sentence_count();
    my $index = $params{sentence_index};
    if ($total > 0) {
        return ($total - $index) / $total;
    } else {
        return undef;
    }

    my %params = @_;
    my @sents = $params{document}->get_sentences();
    my $sent = $params{sentence};
    my $first_sent = $sents[0];

    return GetLexSim($sent, $first_sent);