Q:

Select element by class name A when next to another element of class name B

I have a load of <div>'s, all with the class name of 'a'. 
Occasionally, mixed inbetween these div's I have a  with the class name of 'b' (these 'b' elements are basically category headers within my listing). 
<div class='a'>item 1</div>
<div class='a'>item 2</div>
<div class='a'>item 3</div>

<div class='b'>category header</div>

<div class='a'>item 4</div>
<div class='a'>item 5</div>

<div class='b'>another category header</div>

<div class='a'>item 6</div>
<div class='a'>item 7</div>

Class 'a' has a border-bottom:1px dotted black;
However, if one of these div's with class 'a' is immediately followed by a div of class 'b' (a category header), I don't want class 'a' to have the dotted bottom border, so I want to set it to border:0;
So, in the example above, I want to remove the dotted border-bottom style property from items 3 and 5 (as they are followed immediately by one of the category divs). 
What css selector would accomplish this? 

A:

I didn't realise that I couldn't select further down the DOM.  My bad. 
So, I've basically flipped my logic around a bit so 'a' has a border-TOP (instead of BOTTOM), unless it comes straight after a div with class 'b'.  This has given me the desired result.