Adding Builtin ``Undefined`` ============================ ``Undefined`` is one of the easier kinds of Builtin's to add because: it is simply a Built-in Symbol, not a Function. Builtin Symbols are pretty limited in what they do. Basically a built-in Symbol is a name in the ``System`` namespace with some properties. Also, there are existing modules of Built-in Symbols that we can use as a model here. Below are the steps in order that were used to create ``Undefined``: .. toctree:: :maxdepth: 1 where whatis similar skeletal checking completing testing pytest