A "Secondary Section" is a named appendix or a front-matter
section of the Document that deals exclusively with the
relationship of the publishers or authors of the Document
to the Document's overall subject (or to related matters)
and contains nothing that could fall directly within that
overall subject. (Thus, if the Document is in part a textbook
of mathematics, a Secondary Section may not explain any
mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of
legal, commercial, philosophical, ethical or political
position regarding them. [FDL v.1.2]
This is a pretty restrictive condition on what can be dubbed Invariant.
(It seems to have been crafted in order to describe the GNU manifesto.)
The goldfish ode pretty clearly would not satisfy it.