Whitelist preceding siblings

Hi,

I need to select elements (using XPath) that are preceded by sibling elements
that are not in a given whitelist. That is, I need to find elements named x,
whose preceding siblings (if any) are all named a, b or c, in any order, and
all optional, but nothing else. So the following is a match:

<c/>
<b/>
<x/>

And so is this:

<x/>

But not this one:

<c/>
<BLOCKER/>
<a/>
<x/>

I've tried something like the following with no luck:

<xsl:template match="x"> ...
<xsl:template match="x[preceding-sibling::*][not(self::a or self::b or
self::c)]"> ...

Any ideas?
Regards,
/Fredrik