DME File - Line Count in the Header

The requirement is to have a line total (counter) in the header (NOT the trailer) of the DME file.

I understand that you can use 'reference to tree node' or 'aggregation' to count elements above the line total, but how can this be achieved when the total needs to be displayed above the line items?

The DME should look like this:

HEADER total lines = 3

001 PAYMENT (including many open items)

002 PAYMENT (including many open items)

003 PAYMENT (including many open items)

TRAILER total lines = 3