preceding A or B which comes first

<B></B>
<A></A>
<A></A>
<B></B>
<A></A>
<A></A>
<B></B>
<C></C>
<A></A>

At <A>, I want to do something based on the condition that <B> precedes 
<A> with no intervening <A>.

<B></B>
<A></A> True
<A></A> False
<B></B>
<A></A> True
<A></A> False
<A></A> False
<B></B>
<C></C>
<A></A> True

I have been working with position but that doesn't seem to be the right 
thing.

Jim Albright
704 843-0582
Wycliffe Bible Translators