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Make the input of `values_` a non-empty list #608

Job	Run time
continuous-integration (8.10.7)	10m 0s
continuous-integration (9.0.2)	10m 38s
continuous-integration (9.2.8)	9m 23s
continuous-integration (9.4.5)	10m 10s
continuous-integration (9.6.4)	11m 46s
continuous-integration (9.8.2)	11m 49s
continuous-integration (9.10.1)	1m 18s
continuous-integration (9.12.1, --allow-newer=base --allow-newer=template-haskell --allow-newer=g...	1m 16s
	1h 6m 20s

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