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 ;
} ;