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(emacs-lisp-intro.info)Understanding rotate-yk-ptr

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`rotate-yank-pointer' in Outline
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   The `rotate-yank-pointer' function looks complex, but as usual, it
can be understood by taking it apart piece by piece.  First look at it
in skeletal form:

     (defun rotate-yank-pointer (arg)
       "Rotate the yanking point in the kill ring."
       (interactive "p")
       (let VARLIST
         BODY...)

   This function takes one argument, called `arg'.  It has a brief
documentation string; and it is interactive with a small `p', which
means that the argument must be a processed prefix passed to the
function as a number.

   The body of the function definition is a `let' expression, which
itself has a body as well as a VARLIST.

   The `let' expression declares a variable that will be only usable
within the bounds of this function.  This variable is called `length'
and is bound to a value that is equal to the number of items in the
kill ring.  This is done by using the function called `length'.  (Note
that this function has the same name as the variable called `length';
but one use of the word is to name the function and the other is to
name the variable.  The two are quite distinct.  Similarly, an English
speaker will distinguish between the meanings of the word `ship' when
he says: "I must ship this package immediately." and "I must get aboard
the ship immediately.")

   The function `length' tells the number of items there are in a list,
so `(length kill-ring)' returns the number of items there are in the
kill ring.