InfoPath Dev

I've got a form with the following layout:

 MyFields:

• Textfield1
• RepeatingSection
• OptionalSection
• Textfield2

What I would like to do is create a sort of autonumbering of the OptionalSection into Textfield2. I've tried to insert the following formula into Textfield2: count(preceding-sibling::*/my:OptionalSection), but it doesn't work.

 Can somebody help me with this problem.

Thanks

See attached - save the file locally, then right click the saved file and select Design to open in design mode.

I used an expression box for this in my sample instead of a field - I'd think given the structure you describe, the XPath would be:

count(../../preceding-sibling::my:RepeatingSection/my:OptionalSection) + 1