;; rember and multirember

;; rember removes the first occurrence of an atom from a list
;; multirember removes all occurrences 

(defun rember (a mylist) 
  (if (null mylist) 
      () 
    (if (eq a (car mylist)) 
	(cdr mylist)
      (cons (car mylist) (rember a (cdr mylist))) 
      )
    ) 
)


(defun multirember (a mylist) 
  (if (null mylist) 
      () 
    (if (eq a (car mylist)) 
	(multirember a (cdr mylist) )
      (cons (car mylist) (multirember a (cdr mylist))) 
      )
    ) 
)


;; firsts and seconds

;; take the list of first elements (or second elements) 
;; from a list of pairs

(defun firsts (mylist) 
  (if (null mylist) 
    () 
 (cons (car (car mylist)) (firsts (cdr mylist))) 
 ) 
)

(defun seconds (mylist) 
  (if (null mylist) 
    () 
 (cons (cadr (car mylist)) (seconds (cdr mylist))) 
 ) 
)