In the SmalltalkLanguage, "nil" is used to represent "no object."

It is...
	* A global symbol that references a unique object that represents "no object."
	* The "nil" instance is unique -- SingletonPattern.
	* The "nil" object is an instance of the class UndefinedObject.  So, if you send it messages that are defined on the UndefinedObject class, it will respond accordingly.
	* Returns ''true'' for the "isNil" message.  (All other classes return ''false.'')

Class Object defines an "isNil" method that answers (returns) ''false.'' UndefinedObject defined an "isNil" method that answers ''true.''
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See also WhatIsNull, TheNilObjectInStronglyTypedLanguages
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CategoryNull