Where does the variable for the left sidebar get set in Drupal 6? I have a Drupal 6 site where I've added a submenu for a new set of products:

products
-old products set 1
-old products set 2
-new products set
--new product 1
--new product 2
--new product 3


that goes to a page where that link then becomes a main menu item:

new products set
-new product 1
-new product 2
-new product 3


I found the code:

$variables['layout'] = 'none';
if (!empty($variables['left'])) {
  $variables['layout'] = 'left';
}


in the template_preprocess_page function in the theme.inc file.

On the main menu page, the left sidebar appears with the menu block in it ($variables['left'] is turned on) but on the submenu page, it is not appearing ($variables['left'] is turned off).  The menu block has both pages listed on the 'page specific visibility settings'.  I'm trying to figure out why the difference but the problem is I cannot find where $variables['left'] ever gets set prior to that in the code.  Any idea ??
I ended up changing the menu block settings from having the 'fixed parent item' at the top level with 'starting level' set to 2nd to having the 'fixed parent item' at 'new products set' with 'starting level' to 1st. Now they both work fine.