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Old codger programmer here: once you figure out the clever hack, give the pm an estimate for how long it would take to implement. The estimate should include the time you have spent so far, the time you will need to finish debugging, the time it will take for QA... Then pad ~20% because they inevitably will ask for a shorter timeline or effort level. Then ask them if they want to add it to the backlog. You look like a hero/genius, they get to feel like they have control.
- thesamim