pr <- as.PowerRelation("12 > 2 ~ 1")
(e1 <- equivalenceClassIndex(pr, c(1, 2)))
# 1
(e2 <- equivalenceClassIndex(pr, c(1)))
# 2
(e3 <- equivalenceClassIndex(pr, c(2)))
# 2
(e4 <- equivalenceClassIndex(pr, c()))
# NULL <- empty set does not exist
stopifnot(all(c(e1,e2,e3,e4) == c(1,2,2)))
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