sub random_subset_X {
  my ($n, $k) = @_; # $n is no. of regions, $k no. of replicates
  my %member;
  while ($k > 0) {
    my $x = int(rand()*$n)+1; # generates random number 1..$n.
    redo if $member{$x};
    $member{$x} = 1;  
  } continue {
    $k--;
  }
  sort keys %member;
}