Returns an object of class Properties, which lists the properties of the Set.
$properties
Set properties include:
empty - is the Set empty or does it contain elements?
singleton - is the Set a singleton? i.e. Does it contain only one element?
cardinality - number of elements in the Set
countability - One of: countably finite, countably infinite, uncountable
closure - One of: closed, open, half-open
The Properties class is essentially a read-only list, with '$' and '[' for accessing elements
but without a '[[' method. It is not exported and therefore can be considered abstract.
Other set accessors:
class(),
elements,
length(),
lower,
max(),
min(),
range(),
traits,
type,
universe,
upper