Nothing type/value aka Unit

[Jonathan Levi]

In D what would be the most appropriate type for a value that 
specifically does not carry a value?

I am working on porting Sodium, a simple FRP library, to D (or 
making one like it).  In FRP, events (aka event occurrences) must 
contain 1 and only 1 value.  So when there is no appropriate 
value, they make its value what they call a Unit.

For the java version of Sodium they define the unit like `enum 
Unit { UNIT }`.  In Haskell when this thing is needed `()` is 
used (an empty tuple).

Is there a more appropriate way to do this in D?  Perhaps in 
Phobos?

What makes more sense then `enum Unit { unit }` to me is `struct 
Unit {}; enum Unit unit;`.