Menu output in Drupal 7

Hi to all!
I've got a trouble: I don't understand how menu output in Drupal 7 can be rewritten?
The standard menu code is like this:

<ul class = "menu">
    <li class = "first leaf"><a href = "#"> link 1</a></li>
    <li class = "leaf"><a href = "#">link 2</a></li>
     ...
   <li class = "last leaf"><a href = "#">link n</a></li>
</ul>

...but I need to do something like that:

<ul class = "menu">
    <li class = "separator">
    <li class = "first leaf"><a href = "#"> link 1</a></li>
    <li class = "separator">
    <li class = "leaf"><a href = "#">link 2</a></li>
     ...
   <li class = "last leaf"><a href = "#">link n</a></li>
</ul>

it mens, add <li class = "separator"> to menu. How to do it? Please, help, I really need it!