<a class="current" ... > for all parent menu Items

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I'm wanting to append the class "current" to all parent menu items of a given sub page when viewed.

eg.

Home
Articles (current)
- Article (current)
- Page 1 (current)
- Page 2
- Article 2
- Page 1
- Page 2
Downloads
- Category 1
- Category 2
Contact Us

Hope this explains what I mean, any ideas appreciated.

-Dave