WordPress MU- How to allow only registered users to comment?

If you want to limit comment posting to those registered on your blog the add this code to wp-comments-post.php around line 25:
$user = wp_get_current_user();

if( get_option(‘comment_registration’ ) && ( false == $user->ID || false == $user->has_cap( “level_0″ ) ) ) {
wp_die( __(‘Sorry, you must be logged in and registered on this blog to post a comment.’) );
}

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