sub recurse_postorder {
  my $self=shift;
  my $debug=shift;
  my @list;
  my $recurser;

  $recurser = sub {
    my $node=shift;
    my $depth=shift;
    print ("\t"x$depth)."$node->{value}\n" if $debug;
    $recurser->($node->{left},$depth+1);
    $recurser->($node->{right},$depth+1);
    push @list,$node->{value};
  };
  $recurser->($self->{root},0);
  return \@list;
}