Need test for preceding-sibling

Hi,

I have a structure like this:

<a1/>
<a2/>
<a1/>
<a2/>
<b/>
<c/>
<a2/>
<a1/>
<a2/>

I need to ensure that each <a2> has a preceding <a1>, although there may be
intervening tags.  I know that

	name(preceding-sibling::*[1]

gives me the name of the first preceding sibling, but how do I construct a
test that gives me only the first preceding <a1> or <a2> tag?

Thanks,
Dave