[jQuery] Is $(this) the first child?

[warren.wessels]

Hello,

I've just started using JQuery, and had a quick question that I don't
seem to be able to answer by reading the 1.2 API documentation.

I have a DOM snippet like:

<ul>
 <li>something</li>
 <li>another thing</li>
 <li>a third</li>
 <li>a final thing</li>
</ul>

And on mouseover, using the $("li").mouseover event, I am adding a
class to the li using $(this).addClass(...).  However, I don't want to
add the class if the li under the mouse is the first li in the ul...
The predicate:

 if( $(this).prev() ) { ... }

doesn't seem to work as I'd expect (i.e. if this has a previous
sibling), and neither does

 if( $(this).is("ul li:first") ) { ... }
or
 if( $(this).is("ul li:eq(0)") ) { ... }
or
 if( $(this).is($(this).parent().children()[0] ) { ... }

Can anyone give me some pointers on how to "detect" that the element
selected is in a certain position in the sibling chain?

Thank you!

Warren