sub ext_s {        # Returns characters in sorted order
  my ($s) = @_ ;
  my %h ;
  @h{split(//, $s)} = undef ;
  return join('', sort keys %h) ;
} ;

sub ext_o {        # Returns characters in original order
  my ($s) = @_ ;
  my @h ;
  return join('', grep { !$h[ord($_)]++ } split(//, $s)) ;
} ;

##</code><code>##

sub ext_dw {
  my ($s) = @_ ;
  my %h ;
  $h{$_} = undef foreach split(//, $s) ;
  return join('', sort keys %h) ;
} ;

sub ext_cn {
  my ($s) = @_ ;
  my %h ;
  $s =~ s/(.)/$h{$1}++?'':$1/ge ;
  return $s ;
} ;