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class Foo
  
  def self.var
    @@variable
  end
  
end

Foo.var # => 
# ~> -:4:in `var': uninitialized class variable @@variable in Foo (NameError)
# ~>  from -:9

class Bar
  @@variable = nil
  
  def self.val
    @@variable
  end
end

Bar.val # => nil