require 'set'
s = Set[] # start with an empty set
s << 1 # => #
s.add 2 # => #: add is a synonym for <<
s << 3 << 4 << 5 # => #: can be chained
s.add 3 # => #: value unchanged
s.add? 6 # => #
s.add? 3 # => nil: the set was not changed