How to know that line is selected in a transaction

Hi

I need to validate that if and line item has an attribute (Part Custom Field28) with value "true" (string) must appear an error message that prevent delete the line item when I do clic on Remove Transaction Line.

I have a validation rule in transaction line that fire with the action remove transaction line, in the condition I have that if _system_selected_document_number equals true continue with the validation and in the validation I have that if the attribute value start with true show an error message in top, but this is not working.

How can I validate that the line item is selected in simple condition or in advanced condition using BML?