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(emacs-lisp-intro.info)kill-rng-yk-ptr last elt

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Enter node , (file) or (file)node

Pointing to the last element
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   The final question is, what happens if the `kill-ring-yank-pointer'
is set to the _last_ element of the kill ring?  Will a call to
`rotate-yank-pointer' mean that nothing more can be taken from the kill
ring?  The answer is no.  What happens is different and useful.  The
`kill-ring-yank-pointer' is set to point to the beginning of the kill
ring instead.

   Let's see how this works by looking at the code, assuming the length
of the kill ring is 5 and the argument passed to `rotate-yank-pointer'
is 1.  When the `kill-ring-yank-pointer' points to the last element of
the kill ring, its length is 1.  The code looks like this:

     (% (+ arg (- length (length kill-ring-yank-pointer))) length)

   When the variables are replaced by their numeric values, the
expression looks like this:

     (% (+ 1 (- 5 1)) 5)

This expression can be evaluated by looking at the most embedded inner
expression first and working outwards:  The value of `(- 5 1)' is 4;
the sum of `(+ 1 4)' is 5; and the remainder of dividing 5 by 5 is
zero.  So what `rotate-yank-pointer' will do is

     (setq kill-ring-yank-pointer (nthcdr 0 kill-ring))

which will set the `kill-ring-yank-pointer' to point to the beginning
of the kill ring.

   So what happens with successive calls to `rotate-yank-pointer' is
that it moves the `kill-ring-yank-pointer' from element to element in
the kill ring until it reaches the end; then it jumps back to the
beginning.  And this is why the kill ring is called a ring, since by
jumping back to the beginning, it is as if the list has no end!  (And
what is a ring, but an entity with no end?)