adjust and add "docs" to img-eval.rkt for Matthias's consumption

collects/teachpack/2htdp/scribblings/img-eval.rkt

@@ -1,9 +1,34 @@
 #lang racket/base
 (require scribble/eval)
-(provide make-img-eval)
+(provide make-img-eval
+         set-eval-handler-to-tweak-images)
+
+#|
+
+make-img-eval : -> (any/c . -> . any)
+  calls @racket[make-base-eval] to create
+  an evaluator, and then initializes it with
+  @racket[set-eval-handler-to-tweak-images].
+
+
+set-eval-handler-to-tweak-images : (any/c . -> . any) -> void?
+
+  loads @racketmodname[2htdp/image] and @racketmodname[lang/posn]
+  into the given evaluator and then sets its eval-handler to
+  increase the bounding box for 2htdp/image images by 1.
+  
+  It does this only for images that the result of evaluation
+  directly (not for example, when lists that contain images
+  are the result of evaluation).
+
+|#
 
 (define (make-img-eval)
   (define img-eval (make-base-eval))
+  (set-eval-handler-to-tweak-images img-eval)
+  img-eval)
+
+(define (set-eval-handler-to-tweak-images img-eval)
   (interaction-eval #:eval img-eval (require 2htdp/image))
   (interaction-eval #:eval img-eval (require lang/posn))
   
@@ -24,6 +49,4 @@
             i))
       (current-eval
        (λ (exp)
-         (adjust-image exp (ce exp))))))
-  
-  img-eval)
+         (adjust-image exp (ce exp)))))))