Generic Function Greatest (4 methods)
On a set, returns two values: the greatest member of the set and
true; on a bag, returns three values: the greatest member of the bag, its
multiplicity, and true; on a map, also returns three values: the greatest key
of the map, its value, and true. If there is not a unique greatest member,
chooses one of the greatest members arbitrarily. Returns
nil if the
collection is empty.