test_that("cardinality works", {
    rel <- list(A = letters[1:3], B = letters[1:2])
    TS <- tidySet(rel)
    card <- cardinality(TS, "A")
    expect_equal(card[1, 2], 3)
    expect_equal(colnames(card), c("sets", "cardinality"))
    card <- cardinality(TS)
    expect_equal(colnames(card), c("sets", "cardinality"))
    expect_equal(nrow(card), 2L)
})