The current_motherpage class

I want to add a ” current_motherpage” class in the first level

• of a subpage with “current_page_item”, like this:

<li class=”pagenav”>

<li class=”page_item current_motherpage”>bla bla bla

<li class=”page_item”>bla bla bla

<li class=”page_item current_page_item”>bla bla

Anyone know how I obtain this?
Thank you.