Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Checkpoint. Find the slice that needs work first. Write
worst_slice(cases) that computes accuracy per slice and returns the
summary string for the lowest-scoring slice:
worst slice: french (0.00)
(accuracy formatted to 2 decimals). If there's a tie, return the one that appears first.