>>17
Here's the exercise in common lisp

(defun ex2.20 (a &rest b)
  (cons a (remove-if-not ((lambda (x) (if (oddp x) #'oddp #'evenp)) a) b)))

&rest is equal to '.'
scheme doesn't have remove-if-not so you can try to implement that

You must not return '() if other-values is an empty list. You must return the results or a.

For example,

(same-parity 1) ==> (1)
(same-parity 1 2 3) ==> (1 3)

Don't know how to use spoilers but here is same-parity in scheme:
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(define (same-parity a . b)
  (define (same-parity-help f a b)
    (if (null? b) a
    (same-parity-help f (if (f (car b)) (cons (car b) a) a) (cdr b))))
  (cons a (reverse (same-parity-help (if (odd? a) odd? even?) '() b))))