When the plain question is the right one
Why it matters Once you have seen reframes win, every problem starts to look like it needs one. Most do not. Checking is the skill, not reframing.
The retention dashboard shows which customers ordered yesterday and again today. A busy day has about 300 orders. A colleague fresh from this module suggests "changing the unit": sort both days by customer and walk the two lists side by side, so no sets are built.
Run through the four questions from the first lesson. Is the opposite question cheaper? Is there a smaller unit? Is there a frequent operation to move work away from? Does the caller use less than the whole answer? If every answer is no, write the plain version.
Write repeat_buyers(yesterday, today). Each argument is a list of order dicts with a "customer" key (a customer may order several times a day). Return the sorted list of customers with at least one order on both days, each once. Rule: your whole code may have at most 60 syntax nodes (code_size). Days have about 300 orders.
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