Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the scope and closure bugs pattern
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
What went wrong: the last line uses bil, a typo for bill. Python cannot find a variable called bil.
The fix: return bill + tip
Remember: NameError shows the exact name it could not find. Compare it letter by letter.
The pattern behind it: scope and closure bugs.
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