sub perms {
  my @arr = @_;
  my $m = @arr;
  my $n = 0;
  {
    print "@arr\n";
    ++$n;
    my ( $i, $d ) = offsets( $n );
    last if $i > @arr;
    my $j = @arr - $i;
    @arr[ $j+1..$#arr ] = reverse @arr[ $j+1..$#arr ];
    @arr[ $j, $j+$d ] = @arr[ $j+$d, $j ];
    redo;
  }
}