l <- list(A = "1",
B = c("1", "2"),
C = c("2", "3", "4"),
D = c("1", "2", "3", "4")
)
TS <- tidySet(l)
union_closed(TS)
union_closed(TS, sets = c("A", "B", "C"))
union_closed(TS, sets = c("A", "B", "C", "D"))
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