How to find out if the preceding sibling is a PI

Hi all,

I have a document that looks something like this:

<?xm-insertion_mark_start author="Nadia Swaby"
time="20051202T111856-0500"?>
<design.consideration></design.consideration>

I need to do a test to find out if the sibling immediately preceding the
design.consideration element is an xm-insertion_mark_start PI.

Basically, I need something like
preceding-sibling::*[position() = 1 and
processing-instruction('xm-insertion_mark_start')]
but that statement doesn't return anything.

Is there any way of doing this?

Thanks

Nadia