(let ((a 5))
  (let ((fun (lambda (x) (max x a))))
    (let ((a 10)
          (x 20)
         )
         (fun 1)
     )
  )
)
;((lambda (formals) (body))  values_for_formals)
;((lambda (x)       (+ x x)) 4)  ===>  8
((lambda (a)
   ((lambda (fun)
     ((lambda 
        (a x)    ;formals
        (fun 1)  ;body
       ) 10 20   ;values for (a x)
     )) (lambda (x) (max x a)) ;value for fun
   )
 ) 5 ;value for a
)

;The scope of let DOES NOT include its arguments.
(let ((a 1) (b 2))
  (let ((b 3) (c (+ a b)))
    (let ((b 5))
      (cons a (cons b (cons c '())))
    )
  )
)

;The scope of letrec DOES include its arguments.
(let ((a 1) (b 2))
  (letrec ((b 3) (c (+ a b)))
    (let ((b 5))
      (cons a (cons b (cons c '())))
    )
  )
)


((lambda (a b)
  ((lambda (b c)
    ((lambda (b)
      (cons a (cons b (cons c '())))
     ) 5
    )) 3 (+ a b)
  )) 1 2
)