1 ;; The first three lines of this file were inserted by DrScheme. They record metadata
2 ;; about the language level of this file in a form that our tools can easily process.
3 #reader(lib "htdp-intermediate-reader.ss" "lang")((modname 20.2.3) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp")))))
4 filter1 : (X Y -> boolean) (listof X) Y -> (listof X)
5 
6 (define (filter1 rel-op alon t)
7   (cond
8     [(empty? alon) empty]
9     [(rel-op (first alon) t) 
10      (cons (first alon)
11        (filter1 rel-op (rest alon) t))]
12     [else
13       (filter1 rel-op (rest alon) t)]))