https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19002 shows a place where a
consumer of gnupg's machine-parseable forms expected that gpg would always
produce the exact same number of fields.
This is clearly a bug with their code, but it would be useful to describe what
sorts of changes users of machine-consumable data should be able to cope with.
- The currently defined fields are: […] Note that in the future, new fields may be added to the end of the line.
- some fields of some data types are currently unpopulated or have no specific
definition. Future versions may define and/or populate these fields.
Implementers consuming this output should be prepared to ignore fields that they
don't know about.