(defun filter-in (pred the-list)
	(cond
		((null the-list)
			()
		)
		((funcall pred (car the-list))
			(cons (car the-list) (filter-in pred (cdr the-list)))
		)
		(T
			(filter-in pred (cdr the-list))
		)
	)
)



Demo:

Break 1 [51]> (filter-in #'evenp '(1 2 3))
(2)
Break 1 [51]> (filter-in #'evenp '(1 2 3 4 3))
(2 4)
Break 3 [63]> (filter-in #'singletonp '((two) (three four) (5))))
((TWO) (5))



(defun filter-out (pred the-list)
	(cond
		((null the-list)
			()
		)
		((funcall pred (car the-list))
			(filter-out pred (cdr the-list))
			
		)
		(T
			(cons (car the-list) (filter-out pred (cdr the-list)))
		)
	)

)



Demo:

Break 5 [65]> (filter-out #'singletonp '((two) (three four) (5))))
((THREE FOUR))

Break 6 [67]> (filter-out #'evenp '(1 2 3 4 3))
(1 3 3)

Break 6 [67]> (filter-out #'evenp '(1 2 3))  
(1 3)