docs improvements

pkgs/plai/scribblings/mutator2.scrbl

@@ -268,6 +268,39 @@ of random mutators:
 
 }
 
+Example:
+@racketblock[(save-random-mutator "tmp.rkt" "mygc.rkt" #:gc2? #t)]
+will write to @filepath{tmp.rkt} with a program like this one:
+@codeblock{
+#lang plai/gc2/mutator
+(allocator-setup "mygc.rkt" 200)
+(define (build-one)
+  (let* ((x0 1)
+         (x1 (cons #f #f))
+         (x2
+          (lambda (x)
+            (if (= x 0)
+              x0
+              (if (= x 1) x0 (if (= x 2) x1 (if (= x 3) x1 x0))))))
+         (x3 1)
+         (x4 (cons x3 x3))
+         (x5 (lambda (x) (if (= x 0) x4 (if (= x 1) x1 x2)))))
+    (set-first! x1 x2)
+    (set-rest! x1 x3)
+    x5))
+(define (traverse-one x5) (= 1 (first (x5 0))))
+(define (trigger-gc n)
+  (if (zero? n) 0 (begin (cons n n) (trigger-gc (- n 1)))))
+(define (loop i)
+  (if (zero? i)
+    'passed
+    (let ((obj (build-one)))
+      (trigger-gc 200)
+      (if (traverse-one obj) (loop (- i 1)) 'failed))))
+(loop 200)
+}
+
+
 @defproc[(find-heap-values [input (or/c path-string? input-port?)])
          (listof heap-value?)]{
   Processes @racket[input] looking for occurrences of @racket[heap-value?]s in