NestedLoops(
    [ 
      ( sub {
        my $prefix = join '', @x[@_];
        [ grep !$seen{$prefix . $x[$_]}++, ($_ ? $_ + 1 : @_) .. $#x ];
      } ) x $n,
    ],
    sub {
      my $ssx = join '', @x[@_];
      $count += (matchss($ssx, $y) or return) * matchss($ssx, $x);
    },
  );