#lang racket

(define a 5000)
(define b 600)
(define c 70)

(define (help) "help")

(define (f x)
  (define (help) (+ x a b c))
  (define a 1000)
  (define b 200)
  (define c 30)
  (help))

(f 4)

; 1. Loops to evaluate the exp in tail position of each pair.
; 2. Loops to create locations (making name available), assigning the matching exp value.
; 3. Evaluates the body.
; Duplicate identifiers are illegal.
(let 
    ((f c)
     (c 999)
     (d c)
     (e b))
  (list c d e f))

; 1. Loops over the pairs by evaluating the exp, create location, assigning value.
; 2. Evaluates the body.
; Duplicate identifiers are legal.
(let*
    ((f c)
     (c 999)
     (d c)
     (e b))
  (list c d e f))

; 1. Loops to create location for name in head position of each pair, 
;    assigning each the value #<undefined>.
; 2. Loops over pairs to evaluate exp in tail position of each (new names are available),
;    then assign value to name.
; 3. Evaluates the body.
; Duplicate identifiers are illegal.
(letrec
    ((f c)
     (c 999)
     (d c)
     (e b))
  (list c d e f))