[FOM] Kripke's outline of a theory of truth

[Timothy Y. Chow]

I know I have asked this question before, but I don't remember whether it 
was on FOM, and more importantly, I can't seem to find where I stashed the 
answer in my own files.

In Kripke's "Outline of a Theory of Truth," he makes the following 
comment, "To my surprise, I have found that the problem of defining the 
languages of transfinite level presents substantial technical difficulties 
which have never seriously been investigated."  He then goes on to say 
that he has some partial results but doesn't go into detail.

What difficulties was Kripke talking about and have they been surmounted 
by subsequent researchers?

Tim