sub kramba {
    my %count;
    my $count;
    $count = sub {
        my( $string) = @_;
        my $length = length( $string );
        
        if ($length < MIN) {
            $count{$_} == 1 and delete $count{$_} for keys %count;
            return \%count;
        }
        
        for my $l (MIN..$length) {
            my $s = substr( $string, 0, $l );
            $count{ $s } += 1;
        }
        $count->( substr( $string, 1 ) );
    };
    $count->(shift);
    for (keys %count) {
        delete $count{$_} unless
          $count{$_} >= 2 and length($_) >= MIN;
    }
    \%count;
}