Welcome to Racket v7.9.0.3 [cs].
> (define x 15)
> (set! x 12)
> x
12
> (let ((x 15))
    (set! x (add1 x))
    x)
16
> (let ((x 15))
    (begin 
      (set! x (add1 x))
       x))
16
> (let ((x 5))
    (let ((cl (lambda (_) x)))
      (cl 1000)))
5
> (let ((x 5))
    (let ((cl (lambda (_) x)))
      (cl 1000)))
5
> (let ((x 5))
    (let ((cl (lambda (_) x)))
      (let ((x 12))
        (cl 1000))))
5
> (let ((x 5))
    (let ((cl (lambda (_) (+ 3 x))))
      (let ((x 12))
        (cl 1000))))
8
> (let ((x 5))
    (let ((cl (lambda (_) (+ 3 x))))
      (let ((x 12))
        x)))
12
> (let ((x 5))
    (let ((cl (lambda (_) (+ 3 x))))
      (begin 
        (set! x 14)
        (cl 1000))))
17
> (let ((x 5))
    (let ((cl (lambda (_) (+ 3 x))))
      (let ((x 120))
        (begin 
          (set! x 14)
          (cl 1000)))))
8
> (begin 
    (set! x (add1 x))
    (set! y (+ x 5))
    (+ x y))
; set!: assignment disallowed;
;  cannot set variable before its definition
;   variable: y
;   in module: top-level
; [,bt for context]
> x 
13
> (define b (box x))
> b
'#&13
> (box? b)
#t
> (unbox b)
13
> (set-box! b 99)
> (box? b)
#t
> (unbox b)
99
> x
13
> (let ((x 12))
    (let ((subr (lambda (y) 
                  (begin (set! y (* y y))
                         1000))))
      (begin
        (subr x)
        x)))

12
> (begin 12 13 14 15)
15
> (begin (+ 2 3) (+ 4 5) (* 6 7))
42
> (random 15)
7
> (random 7)
1
> ((lambda (f)  
    (+ (f 5) (f 5)))
   (lambda (n) (random n)))
3
> ((lambda (f)  
    (+ (f 5) (f 5)))
   (lambda (n) (random n)))
6
> ((lambda (f)  
    (+ (f 5) (f 5)))
   (lambda (n) (random n)))
4
> ((lambda (f)  
    (+ (f 5) (f 5)))
   (lambda (n) (random n)))
1
> 
[jhemann:21SP-CS2800] 21:33:53$