activesupport/lib/active_support/core_ext/object/inclusion.rb


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class Object
  # Returns true if this object is included in the argument. Argument must be
  # any object which respond to +#include?+. Usage:
  #
  #   characters = ["Konata", "Kagami", "Tsukasa"]
  #   "Konata".in?(characters) # => true
  #
  def in?(another_object)
    another_object.include?(self)
  end
end