use List::Util qw(min max reduce);
sub score {
    my($word) = @_;
    # uses global %good, %bad, $ngood, $nbad
    my $g = 2 * ($good{$word} || 0);
    my $b = $bad{$word} || 0;
    unless($g + $b < 5) {
       return max(0.01, min (0.99, min (1, $b / $nbad)/
         (min(1, $g / $ngood) + min (1, $b / $nbad))));
    }
    # otherwise: return undef
}

sub prob {
    my @probs = @_;
    my $prod = reduce { $a * $b } @probs;
    return $prod / ($prod + reduce { $a * $b } map { 1 - $_ } @probs);
}