How i can record no change by INTERVAL_EVENT element?

Good afternoon, dear colleagues. In data structures information model Data Structures Information Model (6.1.5 Change Data) i see examples how i can record increase\decrease or change of variable, but how I can record no change in variable?
In my opinion i can it only by math_function actual. But maybe there are other ways.
P.S. In ADL i can record 0 in field with increase/decrease math_function, but i dont know is it bug. (see Pic.