Texinfo-mode: filling breaks @dfn{}

[Alan Mackenzie <none@example.invalid>]

Emacs 21.1

In a file.texi, type in a @dfn{}, such that the term being elucidated
makes a line too big.  Then fill the paragraph with M-q (or let
auto-fill-mode do this).  For example:

...............
line, and which begins in column 0.  (By @dfn{continuation of the
previous line} is meant that the previous line is syntactically
...............

The filling fails to put the necessary \ to continue the @dfn onto the
next line.  That this is erroneous can be seen by the error messages
subsequently given by makeinfo:

cc-mode.texi:5114: Misplaced {.
cc-mode.texi:5114: Misplaced }.
cc-mode.texi:5114: @code missing close brace.

-- 
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
(like "aa"), remove half of them (leaving, say, "a").