[antlr-interest] Value types - can't be set to null in C#

[Johannes Luber]

Johannes Luber schrieb:
> The default is, as you see, null. Adding new types to this structure is
> an open-ended task. So we'd need a new way to tell ANTLR, that there is
> a special case. Maybe "prog: dt(DateTime.MinValue)=datetime"? Ter?

I suppose, this fix will also allow $(default)reference, as labels won't
be the single place, where value types in C# can appear.

Johannes