Test processing instruction as first preceding sibling

Hi guys,

        I have a doubt about testing preceding-sibling
processing-instruction.
        I have this xml:

       <root>
           <child1>
                   child 1
           </child1>
           <?proc?>
           <child2>
                  child 2
          </child2>
     </root>

           And I need to know if child2 has the first
preceding-sibling as a processing-instruction. In this case it should
return true.

            The xml can come like this too:

     <root>
           <child1>
                   child 1
           </child1>
           <child2>
                  child 2
          </child2>
      </root>
        In this case it should return false

and like this:

      <root>
          <?proc?>
           <child1>
                   child 1
           </child1>
           <child2>
                  child 2
          </child2>
      </root>

and here false too.

I tryed something like this

preceding-sibling::*[1]/name()|preceding-sibling()[1]/name = 'proc'

but it didn4t work.

Can anyone help me??

Thanks!!

Alexandre