[MlMt] How do I only show unread messages in smart mailboxes

[Jon Nall]

I want to create a smart mailbox that (among other criteria) only 
contains unread messages. I perused all of the criteria but with the 
exception of possibly #Is-New-Message (which I couldn't find any docs 
on), I came up empty.

Any advice on how to create a smart mailbox that contains only unread 
mail?

Thanks!
nall.