Is there an inverse 'member?' method in ruby?

Not in ruby but in ActiveSupport:

characters = ["Konata", "Kagami", "Tsukasa"]
"Konata".in?(characters) # => true

You can easily define it along this line:

class Object
  def is_in? set
    set.include? self
  end
end

and then use as

8.is_in? [0, 9, 15]   # false
8.is_in? [0, 8, 15]   # true

or define

class Object
  def is_in? *set
    set.include? self
  end
end

and use as

8.is_in?(0, 9, 15)   # false
8.is_in?(0, 8, 15)   # true

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