Sentinel values
===============

Sentinel values are basically a way to combine an enumeration (that is,
`only`-values in SLUL) with a reference to an object.

There are (at least) two ways to implement sentinel (constant singleton)
values:

* Per type definition
* Per type reference


Per type definition
-------------------

Example:

    # Class "File"
    sentinels
        Invalid "" None

    String filename
    ?Int fd

    constructor new
        String filename
        ?Int fd
        ...


Example of "per reference definition"
-------------------------------------

Example:

    # Class "ErrorKind"
    only
        NotFound
        ConnectionError
        ResponseError

    # Class "ExampleClass"
    func get_value
        ...
    return
        SomeObject (or ErrorKind) file

I think "per reference definition" might be better?

This could also be extended to allowed for multiple enums:
(but this will only work if all but the first are closed enums)

    SomeObject (or FileErrorKind NetErrorKind)

Or specific enum items:

    SomeObject (or ErrorKind.NotFound)

And it could be used to replace option-types/None:
(perhaps `?` could be a shorthand for it?)

    SomeObject (or Optional.None)


Limitations
-----------

The enum values must not collide with valid pointer vlues on ANY
current or future supported platform.