Export `posn' contract to the HtDP teaching languages.

collects/lang/posn.rkt

@@ -1,10 +1,15 @@
 #lang racket/base
 
+(require lang/private/contracts/contract-syntax)
+
 ;; The posn struct for the teaching languages
-(provide (struct-out posn) make-posn)
+(provide make-posn posn? posn-x posn-y set-posn-x! set-posn-y!
+	 (rename-out (posn-contract posn)))
 
 (struct posn (x y) #:mutable #:transparent)
 
 ;; We define a separate function so tha it has the 
 ;; name `make-posn':
 (define (make-posn x y) (posn x y))
+
+(define posn-contract (contract (predicate posn?)))

collects/lang/private/beginner-funs.rkt

@@ -313,6 +313,7 @@
 	" in a list of pairs"))
 
     ("Posns"
+      (posn contract "contract for posns")
       (make-posn (number number -> posn) "to construct a posn")
       (posn? (anything -> boolean) "to determine if its input is a posn")
       (posn-x (posn -> number) "to extract the x component of a posn")