First Skeletal version
=======================

.. contents::
   :depth: 1
   :local:

Armed with the information above we are now ready to draft the first version of this.

Mathics3 built-in symbols are defined by writing a class which is based off of the class ``Builtin``.

The docstring for this class lists:

* The common name for this, if a corresponding match can be found Wikipedia, use that name and link to it.
* links to the WMA reference we found before.
* If there is a corresponding SymPy, mpmath, or SciPy function, then link to that
* A HTML "definition list" to enclose the definition of each form that can appear
* User-Oriented Examples.

``Undefined`` Title Line
-------------------------

Here, there is no Wikipedia common name, so we'll go with the Mathematica-like description "Undefined symbol or value". The WMA link is put in parenthesis after that.
See the next case study :ref:`Adding Builtin Function ``KroneckerProduct``` for an example where we have Wikipedia and SymPy entries.

To start out then we have:


.. code-block:: python


    class Undefined(Builtin):
        """
        Undefined symbol/value (<url>:WMA: https://reference.wolfram.com/language/ref/Undefined.html</url>)
	...
	"""

The slightly unusual way we add a link with text is via the XML ``<url>`` tag.
Also, due to current limitations in our homegrown documentation system, the title line has to appear as one unbroken line. To accommodate this, We have arrange ``pylint`` to ignore long lines.

Note: please add the class ``Undefined`` in alphabetic order of class name. At the time of writing, ``Undefined`` comes after ``Pi`` and before ``Underflow``.

``Undefined`` Symbol Definition Description
--------------------------------------------

Let us fill out the docstring further, the line above with ``...`` in it.

After the title line, we get to the definition list part where we list the forms that this can appear. Here there is just one form ``Undefined``:

.. code-block:: python

    class Undefined(Builtin):
        """
        Undefined symbol/value (<url>:WMA: https://reference.wolfram.com/language/ref/Undefined.html</url>)

        <dl>
          <dt>'Undefined'
          <dd>a symbol that represents a quantity with no defined value.
        </dl>

    	...
	"""

We use the pseudo XML ``<dt>`` and ``<dl>`` for the definition. Note
though that we don't use the end tags: ``</dt>`` and ``</dd>`` although
we could if we wanted to.

Notice that the word ``Undefined`` appears within single quotes: ``'Undefined'``. This is markup that tags ``Undefined`` as code. A complete list of homegrown markiup can be found in :ref:`Documentation Markup`.

User-Oriented Examples
----------------------

Now we get to the final part of the docstring consists of informative
examples for the Symbol which is getting defined. We will copy an
example found in the WMA documentation for this and add an example
that shows the attributes.

.. code-block:: python


    class Undefined(Builtin):
        """
        Undefined symbol/value (<url>:WMA: https://reference.wolfram.com/language/ref/Undefined.html</url>)

        <dl>
         <dt>'Undefined'
         <dd>a symbol that represents a quantity with no defined value.
    </dl>

    >> ConditionalExpression[a, False]
     = Undefined
    >> Attributes[Undefined]
     = {Protected}
    """
    # ...

In a later we will describe in more detail what the lines with ``>>`` and ``=`` below that mean.

Next:

.. toctree::
   :maxdepth: 1

   checking