require 'set' s = (1..3).to_set # => #<Set: {1, 2, 3}> s.delete 1 # => #<Set: {2, 3}> s.delete 1 # => #<Set: {2, 3}>: unchanged s.delete? 1 # => nil: returns nil when no change s.delete? 2 # => #<Set: {3}>: otherwise returns set