On Mon, Nov 09, 2020 at 11:49:33PM +0100, Christopher Dimech wrote:
> What is the meaning of the '.' in '#1=(nil t always . #1#)?

This means that the last cdr in that list isn't nil, as "decent,
normal lists" (aka "proper lists") have, but a reference to something,
in this case to the list's head (#1#), which you have taken previously,
in #1=. Making the whole thing an Ouroboros [1].

In box-and-pointer-ish, the list '(nil t always) looks like so
(WARNING: psychedelic effects guaranteed when using variable-pitch
fonts. But who does that, anyway?):

  +-----+--+      +--+--+      +--+-----+  
  | nil |  | ---> |  |  | ---> |  | nil |
  +-----+--+      +--+--+      +--+-----+     
                    |            |
                    v            v
                   't          'always

Note how the list ends with a pair whose cdr is nil.

The dot notation allows you to fill both slots of a pair (car
and cdr), thus putting something other than the "link to next"
in a cdr, creating an improper list; the reference notation
(that #...= and #...#) allows you to take a ref at some place
and use it later. That's what your above monster looks like
in box-and-pointer:

        +---------------------------------+
        |                                 |
        v                                 |
  +-----+--+      +--+--+      +--+--+    |
  | nil |  | ---> |  |  | ---> |  |  | ---+
  +-----+--+      +--+--+      +--+--+     
                    |            |
                    v            v
                   't          'always


So if you run *that* list along (by doing cdr), you'll run
in circles, over and over again. That was this device's
intention, anyway, if I followed along correctly.

Cheers

[1] https://en.wikipedia.org/wiki/Ouroboros
 - t