notes:

• rember
  • It takes an atom and a lat as its arguments, and makes a new lat with first occurence of the atom in the old lat removed.
• Use cons to build lists
• firsts
  • The function firsts takes one arguments, a list, which is a null list or containes only non-empty lists. It builds another list composed of the first S-expression of each internal list
• When building a list, describe the first typical element, and then cons it onto the natural recursion.
• insertR
  • (insertR new old lat)
  • It takew three arguments: the atoms new and old, and a lat. The function insertR builds a lat with new inserted to the right of the first occurence of old

(define rember
  (lambda (a lat) 
    (cond 
      ((null? lat) (quote ()))
      (else 
        (cond 
          ((eq? a (car lat)) (cdr a lat)
          (else (cons (car lat) 
                      (rember a 
                              (cdr a lat))))))))))

(define firsts
  (lambda l 
    (cond 
      ((null? l) (quote()))
      (else (cons (car (car l))
                  (firsts (cdr l)))))))

(define insertL
  (lambda (new old lat)
    (cond 
      ((null? lat) (quote ()))
      (else 
        (cond
          ((eq? (car lat) old) (cons new lat))
          (else (cons (car lat) 
                      (insertL new old (cdr lat))))))))

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