What does () mean?

In R2, () was an error.

In R3, () returns the value: UNSET!

So:

>> mold ()
== "unset!"

It's debatable.

We must admit that it does represent a valid expression. This can be seen in functions that allow missing arguments, such as:

>> cd
/C/rebol/3.0
>> cd %..
/C/rebol/

which are valid anywhere if you use parens:

>> (cd)
/C/rebol/3.0
>> (cd %..)
/C/rebol/

So, in the (cd) case, the missing argument for path is given the value UNSET!

Mathematically, () is the trivial case. QED.

Comments?