HOW DO REFINEMENTS HAPPEN?
scores: PCollection[KV[str, int]] = (input
| beam.WindowInto(FixedWindows(2 * 60),
triggerfn=trigger.AfterWatermark(
early=trigger.AfterPeriod(1*60),
late=trigger.AfterCount(1)),
accumulation_mode=ACCUMULATING)
| beam.CombinePerKey(sum))