Rules that Don't Exist

[Bill Seymour]

A tz newbie question:

What happens when switching to a named rule, but no such rule exists?

The Europe/Zurich example in zic.8.txt has an example of that (switches
to Swiss rules in 1894, but there are no Swiss rules before 1940), but
doesn't say what to do.  I can make sense of that by making what seems
to be the reasonable assumption that SAVE is zero and LETTER/S is null.
Is that the intent?

If so, then is it also true that, in America/Chicago, we should use
"CT" rather than "CST" from 1883 Nov 18 12:09:24 to the last Sunday
in March of 1918?  Or is there something I'm missing?

Thanks,

--Bill Seymour