calculating relative position in document order

Hi,

I'd like to be able to determine which element is "closer", in terms of document order, to a given element. eg, given two structures containing siblings:

        <a>
        <b>
        <c>
and
        <b>
        <a>
        <c>

I'd like to detect that the second structure is "out of order". In my fantasy pseudo code, I might express the constraint as:

//c[positionEX(preceding::a[1]) > positionEX(preceding::b[1])]

Where my fantasy positionEX() function returns the absolute position of the argument in the document. 

Am I missing something, or is implementing this operation in real xpath/xslt much more difficult than it seems it should be?

Thanks in advance for any help...

--T