2017 January

;;Define a Scheme function foo which finds all atoms inside an sexpression which pass a given predicate.
;;Examples:
;;(foo number? '(a (2 (c 3) 4)))
;;=> (2 3 4)
;;(foo symbol? '(a (2 (c 3) 4)))
;;=> (a c)
;;(foo symbol? 'a)
;;=> (a)
;;(foo number? 'a)
;;=> ()

(define (foo p? s)
  (set! s (flatten s))
    (cond
      ((null? s) null)
      ((p? (car s)) (cons (car s) (foo p? (cdr s))))
      (else (foo p? (cdr s)))))