Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the logic errors pattern
The wrong operator, an inverted condition, a formula the right way round but backwards.
What went wrong: Python forbids mutating a dictionary while iterating directly over its keys or items view.
The fix: Iterate over a snapshot list of keys: for token in list(sessions.keys())
Remember: In Python, iterate over a copy like list(dict.keys()) when deleting keys during a loop.
The pattern behind it: logic errors.
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