Determining whether no single instance of a specific n

Hi Everyone,

I have the following XML document:

     <a>
	     <b/>
	     <b/>
	     <b/>
     </a>
     <a>
   	     <b/>
	     <b/>
	     <b>
		     <c/>
	     </b>

     </a>
     <a>
	     <b/>
	     <b/>
	     <b/>
     </a>

For each <b/> set, I want to apply a transformation only if no single
instance of <c/> exists. I know how to check whether a node does
exist, but not how to check whether no single instance of the node
exists.

I've tried the following conditional statements without success:

1,
<xsl:when test="not(a/b/c)">

2.
<xsl:when test="count(not(a/b/c)) &gt;= 1">


What conditional statement should I use?

Thanks in advance for any help.

Mark