handle `digits` argument #1929

Job	Run time
R-CMD-check / macOS-latest (release)	4m 53s
R-CMD-check / windows-latest (devel)	6m 4s
R-CMD-check / windows-latest (release)	5m 5s
R-CMD-check / ubuntu-latest (devel)	6m 45s
R-CMD-check / ubuntu-latest (release)	7m 30s
R-CMD-check / ubuntu-latest (oldrel-1)	6m 24s
R-CMD-check / ubuntu-latest (oldrel-2)	3m 59s
R-CMD-check / ubuntu-latest (oldrel-3)	4m 41s
R-CMD-check / ubuntu-latest (oldrel-4)	5m 31s
	50m 52s

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