sub unique (@)
{
    my ($arr) = shift;

    my %x;
    for my $index (0 .. $#$arr)
    {
        my $val = $arr->[$index];
        !defined($val) && do {
            $x{__NOT_DEFINED__} ||= [
                $index,
                undef,
            ];
            next;
        };

        $x{$val} ||= [
            $index,
            $val,
        ];
    }

    map { $_->[1] } sort { $a->[0] <=> $b->[0] } values %x;
}