> While it is still unclear what you exactly mean by "#! directive",

It's not that important what we mean by the word because the concept
isn't that well developed. RnRS only has a small handful of directives.

I'm using it to mean something that doesn't cause a datum to come out of
`(read)`. But a lot of the existing #! does emit a datum, which shows
that "doesn't emit a datum" may not be a good principle for "#!" to have
in RnRS.

> taking the above list as examples, it is clear the shebang lines are no
> directives in this sense and can and should be left out of the discussion.

A shebang line shouldn't emit a datum out of `(read)`, but can benefit
from other processing, so I think it fits fine with directives.

You could say a directive has to affect reader state. A shebang line can
affect reader state (e.g. "#! /usr/bin/env r7rs-script" could switch to
R7RS read syntax) but does not have to. #!nocache already exists and
doesn't affect reader state either.

>     #!key
>     #!optional
>     #!rest
>
> What do you mean by "only valid inside a lambda list"? The reader
> doesn't know the concept of a lambda list.

You're right. In Gambit and Kawa you can type stuff like (eq? #!optional
#!optional) and it works. So these are implemented as self-evaluating
objects.

In Kawa #:foo needs to be quoted explicitly, similar to vector literals
in R6RS. Similarly one could require '#!optional and such. But this is
not reader-level stuff either.