is_tree (is sub-page conditional)

WordPress does not include a function to check if is_subpage. This function provides that for us.

// This function will return true if we are looking at the page in question or one of its sub pages
function is_tree( $pid ) {              // $pid = The ID of the page we're looking for pages underneath
    global $post;                       // load details about this page

	if ( is_page($pid) )
		return TRUE;            // we're at the page or at a sub page

	$anc = get_post_ancestors( $post->ID );
	foreach ( $anc as $ancestor ) {
		if( is_page() && $ancestor == $pid ) {
			return TRUE;
		}
	}

	return FALSE;  // we aren't at the page, and the page is not an ancestor
}