;This function takes a list of arbitrary levels 
;as its argument.
;The function recurses through all levels of the list
;and returns a list of all unique atoms present in the 
;original list.

(define extractUniqueAtoms 
  (lambda (mylist)
    ;In the recursive calls to extractUniqueAtoms_recurse
    ;All unique atoms found are added into atomList which
    ;ultimately is the return value of this function.
    (define atomList '())
   

    (define extractUniqueAtoms_recurse 
      (lambda (mylist)
        (cond 
          ((null? mylist) #t)
          ((atom? (car mylist))
            (if (eq? (memq (car mylist) atomList) #f)
              (set! atomList (cons (car mylist) atomList))
            )
            (extractUniqueAtoms_recurse (cdr mylist))
          )
          ((list? (car mylist))
            (extractUniqueAtoms_recurse (car mylist))
            (extractUniqueAtoms_recurse (cdr mylist))
          )  
        )
      )
    )
    
    (extractUniqueAtoms_recurse mylist)
    (reverse atomList)
  )
)


;============== Unit Tests ======================
(load "atom.scm")
(load "test.scm")
(define msg "extractUniqueAtoms")
(test msg (extractUniqueAtoms '(a b c d)) '(a b c d))
(test msg (extractUniqueAtoms '(a a a a)) '(a))
(test msg (extractUniqueAtoms '(a (b) c d)) '(a b c d))
(test msg (extractUniqueAtoms '(a (b) c b)) '(a b c))
(test msg (extractUniqueAtoms '(a () c d))  '(a c d))
(test msg (extractUniqueAtoms '(a (a) (c (x y) d) z)) '(a c x y d z))