What are the array keys of $content that can be hidden? I see that with drupal 7 theming, we can mark elements of the $content array as hidden.

From http://drupal.org/update/themes/6/7 :

  &lt;?php
    // We hide the comments and links now so that we can render them later.
    hide($content['comments']);
    hide($content['links']);
    print render($content);
  ?&gt;


What other keys always exist? Or if they are dynamically generated, what is the best way to check the hideable content elements?

Thanks

Edit
With the devel module enabled, I found that the information I was looking for roughly corresponds to the information found at the location /node/your_nid/devel/render.
Or using PHP directly, dprint_r(array_keys($content));
Yes, it is dynamical, all modules can add new elements.

Install http://drupal.org/project/devel and then use dpm($content); Note that when you do this inside a page, you will probably only see it after another reload.