I have a conceptual question concerning "HNN extensions".
In the book
B. Chandler / W. Magnus, "The History of Combinatorial Group Theory" (1982)
on page 111, the definition of the HNN extension is given. It is followed
by the remark: "In particular, t (the "stable letter") is always of infinite
order in H (the extended group)."
Is this remark really correct - and if, then why ?