showClass("OptionalInteger")
showClass("OptionalList")
(oc <- getClass("OptionalCharacter"))
stopifnot(isVirtualClass(oc),
isClassUnion(oc),
all(c("NULL", "character") %in% names(oc@subclasses)),
any(extends("NULL") == "OptionalCharacter"),
any(extends("character") == "OptionalCharacter"))
getClass("flint")@slots
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