class Foo; attr_accessor :test; end                                  
#=> nil                                                                  
f = Foo.new                                                          
#=> #                                                     
f.test = "FOO"                                                       
#=> "FOO"                                                                
g = Foo.new                                                          
#=> #                                                     
g.test = "BAR"                                                       
#=> "BAR"                                                                
class << f; def test2; "#{test} 2"; end; end                         
#=> nil                                                                  
f.test2                                                              
#=> "FOO 2"                                                              
g.test2                                                              
# NoMethodError: undefined method `test2' for #