What is ``Undefined``?
-----------------------

In the last section we looked up the documentation for ``Undefined``. What exactly is it?

To find it in the WMA docs we add the name after ``https://reference.wolfram.com/language/ref/`` which gives `<https://reference.wolfram.com/language/ref/Undefined>`_
It is listed as a "Built-in Symbol". In contrast to the other examples and indeed most of the kinds of things you are likely to add, this one is not a function, but a "symbol". The description for this is "a symbol that represents a quantity with no defined value".

Asking in the `Q/A section of mathics-core <https://github.com/Mathics3/mathics-core/discussions/513#discussioncomment-3402878>`_ we learn that
``Attributes[Undefined]={Protected}``.

Another place where you may be able to get information about Mathematica is `StackExchange Mathematica and Wolfram Language <https://mathematica.stackexchange.com/>`_

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