package Node;

BEGIN {
   my $idx = 0;
   require constant;
   constant->import($_ => $idx++) foreach qw(
      IDX_PARENT
      NEXT_IDX
   );
}

##</code><code>##

package ElementNode;

BEGIN {
   our @ISA = 'Node';

   my $idx = __PACKAGE->SUPER::NEXT_IDX();
   require constant;
   constant->import($_ => $idx++) foreach qw(
      IDX_NAME
      IDX_ATTS
      NEXT_IDX
   );
}