RE: typo in accept-process-output (process.c)

[Drew Adams]

> Just a thought: Is there any other (non-math and non-logical) software
> that uses iff in the documentation?

Of course. (Some of the results of googling `iff software documentation' are
examples.)

Also, the Emacs user community includes mostly developers so far, and many
of them produce software, specs, and documentation that are aimed at other
developers. My guess is that many (most?) Emacs users will already be
familiar with "iff", but I could be wrong about that.

On the other hand, I see no special reason to use "iff" when "if and only
if" can be used instead. It could argued that "iff" can be handy for the
first line of a doc string, where space is at a premium, but in most cases
such rigor is not needed in that first line. And of course there is no
hyperlinked glossary for doc strings (either).

Overall, I'd say we don't need to use "iff". In any case, we should not
substitute "if" for "if and only if" in contexts where the distinction is
important.

If we had glossary links with visual cues, then it could be OK to use "iff",
but I see no real reason not to always use "if and only if" instead.

It can be OK to use just "if" informally, at first (say in a doc string
first line), provided that the rest of the text tells the whole truth, and
provided that doing so improves readability (e.g. by sounding less formal).

Rules of thumb:

1. Use "if A then B" or "A implies B" for implication. These are generally
clearer to readers than the equivalents "B if A" and "A only if B".

2. Use "if and only if" for logical equivalence, not "if".

(Even though "A if B" and "A only if B" can confuse readers about the
direction, there is no such problem when both directions are meant.)