R4RS 4.2.2  (letrec <bindings> <body>)  ==>  object

Syntax: <Bindings> should have the form

((<variable1> <init1>) ...),

and <body> should be a sequence of one or more expressions. It is
an error for a <variable> to appear more than once in the list of
variables being bound.

Semantics: The <variable>s are bound to fresh locations holding
undefined values, the <init>s are evaluated in the resulting
environment (in some unspecified order), each <variable> is assigned
to the result of the corresponding <init>, the <body> is evaluated
in the resulting environment, and the value of the last expression
in <body> is returned. Each binding of a <variable> has the entire
LETREC expression as its region, making it possible to define
mutually recursive procedures.

(letrec ((even?
           (lambda (n)
             (if (zero? n)
                 #t
                 (odd? (- n 1)))))
         (odd?
           (lambda (n)
             (if (zero? n)
                 #f
                 (even? (- n 1))))))
  (even? 88))                         ==>  #t

One restriction on LETREC is very important: it must be possible
to evaluate each <init> without assigning or referring to the value
of any <variable>. If this restriction is violated, then it is an
error. The restriction is necessary because Scheme passes arguments
by value rather than by name. In the most common uses of LETREC,
all the <init>s are LAMBDA expressions and the restriction is
satisfied automatically.
