Missing @footnote support in texinfo module

scheme@(guile-user)> (use-modules (texinfo))

scheme@(guile-user)> (use-modules (texinfo serialize))

scheme@(guile-user)> (display (stexi->texi (texi-fragment->stexi "@defun Foo\nLorem Ipsum.@end defun")))

@c %start of fragment

@anchor{defun-Foo}@defun Foo

Lorem Ipsum.

@end defun

@c %end of fragment

scheme@(guile-user)> (display (stexi->texi (texi-fragment->stexi "@footnote{Foo}")))

ice-9/boot-9.scm:1685:16: In procedure raise-exception:

Throw to key `parser-error' with args `(#f "Unknown command" footnote)'.

Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.