Perform a main doc action on multiple selected items in the line item grid

Summary:

How to set boolean value to True through _system_selected_document_number for multiple selected products in the line item grid while performing a main doc action?

Content (required):

Hi,

I have a sub doc boolean attribute, named p, on the line item grid which is by default "False" and I would like to have the possibility to select multiple line items and when hitting a button on the main doc, the p value should be changed to "True" only for those selected.

In order to achieve this I have created a modify main action, where I added below code snippet in the Advanced Modify - After Formulas.