feat: add test coverage #68

Job	Run time
R Package Checks / lintr / lint	2m 35s
R Package Checks / codecov / test-coverage	38m 9s
R Package Checks / docs-check / docs-check	2m 20s
R Package Checks / check-R-updates / updated-files	3s
R Package Checks / r-cmd-check / macOS-latest (release)	1m 50s
R Package Checks / r-cmd-check / windows-latest (release)	2m 41s
R Package Checks / r-cmd-check / windows-latest (3.6)	4m 50s
R Package Checks / r-cmd-check / windows-latest (4.1)	4m 27s
R Package Checks / r-cmd-check / ubuntu-latest (release)	3m 20s
R Package Checks / r-cmd-check / ubuntu-latest (devel)	2m 49s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-1)	1m 28s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-2)	1m 37s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-3)	2m 3s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-4)	1m 49s
R Package Checks / version-check / version-check	1m 6s
	1h 11m 7s

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