Using predicate with a variable to iterate through nodes in a while loop

Summary

Using predicate with a variable to iterate through nodes in a while loop

Content

We need to process a flat file using NXSD. The file structure contains a lot of invoice headers and invoice details that are in one sequence and not nested, and therefore OIC cannot use a for-each loop to process them.

I'm trying to iterate them by using a count nodes function on the details to populate a variable called $DetailsCount with the number of the details and then use a while loop to iterate through the details and map the fields of each detail element using a predicate with the index which is $Rowcount: nssrcmpr:Read/nsmpr1:BatchRecord/nsmpr1:InvoiceDetails[$Rowcount]/nsmpr1:ORDCOD