Add insert! for 3.25

Could do with some tidying up but seems to work OK.

3_25.rkt

@@ -5,40 +5,44 @@
 (define (make-table)
   (list '*table*))
 
-;;(define (find-record
-
 (define (assoc key records)
   (cond ((null? records) false)
         ((equal? key (caar records)) (car records))
         (else (assoc key (cdr records)))))
 
-(define (table? l)
-  (pair? (cdr l)))
+(define (empty? table)
+  (not (list? (cdr table))))
 
 (trace-define (lookup keys table)
   (cond ((null? keys) (cdr table))
-        ((not (table? table)) false)
+        ((empty? table) false)
         (else
          (let ((subtable (assoc (car keys) (cdr table))))
            (if subtable
                (lookup (cdr keys) subtable)
                false)))))
 
-(define (insert! key-1 key-2 value table)
-  (let ((subtable (assoc key-1 (cdr table))))
-    (if subtable
-        (let ((record (assoc key-2 (cdr subtable))))
+(define (insert! keys value table)
+  (trace-define (insert-subtable! keys subtable)
+    (if (and (not (null? keys))
+             (not (null? subtable)))
+            (let* ((key (car keys))
+                   (record (assoc (car keys) (cdr subtable))))
               (if record
-              (set-cdr! record value)
+                  (if (= (length keys) 1)
+                      (set-cdr! record value))
                   (set-cdr! subtable
-                        (cons (cons key-2 value)
-                              (cdr subtable)))))
-        (set-cdr! table
-                  (cons (list key-1
-                              (cons key-2 value))
-                        (cdr table)))))
+                            (cons (cons key
+                                        (if (= (length keys) 1)
+                                            value
+                                            '()))
+                                  (cdr subtable))))
+              (let ((next-subtable (assoc key (cdr subtable))))
+                (insert-subtable! (cdr keys) next-subtable)))))
+  (insert-subtable! keys table)
   'ok)
 
+
 (#%require (only racket/base module+))
 
 (module+ test
@@ -46,7 +50,8 @@
 
   (test-begin
     (define t (make-table))
-    (insert! 10 10 'a t)
+    (insert! '(10 10) 'a t)
     (check-equal? (lookup '(10 10) t) 'a)
-    (check-equal? (lookup '(10 10 10) t) false))
-  )
+    (check-equal? (lookup '(10 10 10) t) false)
+    (insert! '(20) 'b t)
+    (check-equal? (lookup '(20) t) 'b)))