XPATH expression

Hi,




    Can somebody explain to me what is the meaning of the following 
expression:

a[not(following::x=.)]


    Sample xml:

<B>
    <a>1</a>
    <a>2</a>
    <a>1</a>
    <x>3</x>
    <x>4</x>
    <x>2</x>
</B>

<B>
    <a>3</a>
    <a>1</a>
    <a>2</a>
    <a>3</a>
    <x>5</x>
    <x>4</x>
    <x>1</x>
</B>



Regards,
Razvan