test_that("test combineStrata", {
  expect_identical(
    combineStrata(c("age_group", "sex", "year")),
    list(
      "age_group",
      "sex",
      "year",
      c("age_group", "sex"),
      c("age_group", "year"),
      c("sex", "year"),
      c("age_group", "sex", "year")
    )
  )

  # Expected errors and results for edge cases
  expect_identical(combineStrata(c("sex")), list("sex"))
  expect_error(combineStrata(sex))
  expect_error(combineStrata(c(2, 3)))
  expect_identical(
    combineStrata(c("sex", NA)), list("sex", NA_character_, c("sex", NA))
  )
  expect_identical(combineStrata(NULL), list())
  expect_identical(combineStrata(c("")), list(""))
  expect_identical(combineStrata(c("")[-1]), list())
})