add mutation assignment lesson

10_mutation_assignment.scm

@@ -0,0 +1,34 @@
+;; -*- geiser-scheme-implementation: guile -*-
+
+(define chest 'sword)
+
+chest  ;  sword
+
+(set! chest 'gold) ;  #<unspecified>
+
+chest ;  gold
+
+
+(define (make-countdown n)
+  (lambda ()
+    (define last-n n)
+    (if (zero? n)
+        0
+        (begin
+          (set! n (- n 1))
+          last-n))))
+(define cdown (make-countdown 3))
+
+(cdown) ;  3
+(cdown) ;  2
+(cdown) ;  1
+(cdown) ;  0
+(cdown) ;  0
+
+(define vec (vector 'a 'b 'c))
+vec ;  #(a b c)
+(vector-ref vec 1) ;  b
+(vector-set! vec 1 'boop)
+(vector-ref vec 1) ;  boop
+vec ;  #(a boop c)
+