Source of headnod as copula

[Lynn S. Messing]

Hello,

I read an article or a chapter in a book some time back which discussed
three different types of head nods in ASL. It indicated that one type of
head nod was, in fact, an overt copula. I'd like to lay hands on that text
again; but unfortunately, I can't recall much more about it than that.

If anyone could provide me with citation information, I would be most greatful.

Regards, Lynn Messing  messing at asel.udel.edu