On 2013–08–25 Wolfgang Schuster wrote:

> The \starttexdefintion command looks for the “unexpanded” keyword
> only when you apply also a argument but not when the arguments
> are missing, i.e.

Thanks for the explanation. Is that a bug or expected behaviour?
I think it's odd that

  \unexpanded\def\foo{}

and

  \starttexdefintion unexpanded foo
  \stoptexdefinition

don't behave the same.

Marco