Turns out I was in "INBOX", which is a Folder. And 'K' only works
for Newsgroups, not for Folders.
Does the distinction in Yarn between "News" and "Mail" make sense --
especially having certain facilities (e.g., 'threads') only available
for "News", with other facilities (e.g., 'individual message delete')
being only available for "Mail"?
Given that the code to handle "threads" exists in Yarn, why should
collections of "Mail" (i.e., Folders) *not* have the benefit of that
way of organizing the information?