sub recurse_inorder {
  my $self=shift;
  my @inorder;
  my $sub;
  $sub=sub{ 
            my $node=shift;
            $sub->($node->left) if $node->left;
            push @inorder,$node->value;
            $sub->($node->right) if $node->right;
          };
  $sub->($self);    # Added due to [dragonchild]s eagle eye
  return \@inorder;
}