1-6-1.rkt~

;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname 1-6-1) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
(define WIDTH 100)
(define HEIGHT 200)

; A UFO is a Posn. 
; interpretation (make-posn x y) is the UFO's location 
; (using the top-down, left-to-right convention)

; (no need to redefine ufo since it's already a posn)
 
(define-struct tank [loc vel])
; A Tank is a structure:
;   (make-tank Number Number). 
; interpretation (make-tank x dx) specifies the position:
; (x, HEIGHT) and the tank's speed: dx pixels/tick

 
; A Missile is a Posn. 
; interpretation (make-posn x y) is the missile's place