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 26.4 Iteration mechanisms
 =========================
 
 Scheme has only few iteration mechanisms, mainly because iteration in
 Scheme programs is normally expressed using recursion.  Nevertheless,
 R5RS defines a construct for programming loops, calling `do'.  In
 addition, Guile has an explicit looping syntax called `while'.
 
  -- syntax: do ((variable1 init1 step1) ...) (test expr ...) command ...
      The INIT expressions are evaluated and the VARIABLES are bound to
      their values. Then looping starts with testing the TEST
      expression.  If TEST evaluates to a true value, the EXPR following
      the TEST are evaluated and the value of the last EXPR is returned
      as the value of the `do' expression.  If TEST evaluates to false,
      the COMMANDs are evaluated in order, the STEPs are evaluated and
      stored into the VARIABLES and the next iteration starts.
 
      Any of the STEP expressions may be omitted, so that the
      corresponding variable is not changed during looping.
 
  -- syntax: while cond body ...
      Evaluate all expressions in BODY in order, as long as COND
      evaluates to a true value.  The COND expression is tested before
      every iteration, so that the body is not evaluated at all if COND
      is `#f' right from the start.
 
    Another very common way of expressing iteration in Scheme programs is
 the use of the so-called "named let".
 
    Named let is a variant of `let' which creates a procedure and calls
 it in one step.  Because of the newly created procedure, named let is
 more powerful than `do'-it can be used for iteration, but also for
 arbitrary recursion.
 
  -- syntax: let variable bindings body
      For the definition of BINDINGS see the documentation about `let'
      ( Local Bindings).
 
      Named `let' works as follows:
 
         * A new procedure which accepts as many arguments as are in
           BINDINGS is created and bound locally (using `let') to
           VARIABLE.  The new procedure's formal argument names are the
           name of the VARIABLES.
 
         * The BODY expressions are inserted into the newly created
           procedure.
 
         * The procedure is called with the INIT expressions as the
           formal arguments.
 
      The next example implements a loop which iterates (by recursion)
      1000 times.
 
           (let lp ((x 1000))
             (if (positive? x)
                 (lp (- x 1))
                 x))
           =>
           0
 

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