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