R4RS 4.2.4  (do <vars> <test> <body>)  ==>  object

More specifically, the DO syntax is:

        (do ((<variable1> <init1> <step1>)
             ...)
            (<test> <expression> ...)
          <command> ...)

DO is an iteration construct. It specifies a set of variables to
be bound, how they are to be initialized at the start, and how they
are to be updated on each iteration. When a termination condition
is met, the loop exits with a specified result value.

DO expressions are evaluated as follows: The <init> expressions are
evaluated (in some unspecified order), the <variable>s are bound
to fresh locations, the results of the <init> expressions are stored
in the bindings of the <variable>s, and then the iteration phase
begins.

Each iteration begins by evaluating <test>; if the result is false
(see section see section 6.1 Booleans), then the <command> expressions
are evaluated in order for effect, the <step> expressions are
evaluated in some unspecified order, the <variable>s are bound to
fresh locations, the results of the <step>s are stored in the
bindings of the <variable>s, and the next iteration begins.

If <test> evaluates to a true value, then the <expression>s are
evaluated from left to right and the value of the last <expression>
is returned as the value of the DO expression. If no <expression>s
are present, then the value of the DO expression is unspecified.

The region of the binding of a <variable> consists of the entire
DO expression except for the <init>s. It is an error for a <variable>
to appear more than once in the list of DO variables.

A <step> may be omitted, in which case the effect is the same as
if `(<variable> <init> <variable>)' had been written instead of
`(<variable> <init>)'.

(do ((vec (make-vector 5))
     (i 0 (+ i 1)))
    ((= i 5) vec)
  (vector-set! vec i i))         ==>  #(0 1 2 3 4)

(let ((x '(1 3 5 7 9)))
  (do ((x x (cdr x))
       (sum 0 (+ sum (car x))))
      ((null? x) sum)))          ==>  25
