[racket] Eval, namespaces redefinitions, and (maybe) code-inspectors

Racketeers,

I'm working through eval, namespace, and syntactical macros

My question concerns re-definition of identifiers.

I've worked through three cases below, demonstrating Racket's default 
behavior.

Case 1 shows that identifiers defined at module (or is it top?) level 
cannot be redefined at namespace level

Case 2 shows that identifiers defined at namespace level *can* be 
redefined at namespace level

Case 3 shows that the re-definition allowed in Case 2 can be prevented 
by using namespace introspection

But, is there a better way to prevent re-defining identifiers with 
namespace-level binding?

Can I parameterize the namespace to prevent  re-definition of 
namespace-level variables? Perhaps using code-inspectors?

Thanks very much.

R./
Zack

;===Demonstration Code: (3 Cases) =============================

#lang racket
(require rackunit)
(define-namespace-anchor a)
(define ns (namespace-anchor->namespace a))

;====CASE 1 identifier which is defined at module-level cannot be 
redefined at namespace level=====


(define-syntax-rule (my-define ([id val]))
   (define id val))

(define x 10)

(eval '(my-define ((x 1))) ns)

; ERROR cannot re-define a constant


;CASE 2 identifier defined at namespace level CAN be redefined at 
namespace level

(define-syntax-rule (my-define ([id val]))
   (define id val))


(eval '(my-define ((x 10))) ns)
(check-equal? (eval 'x ns) 10)

(eval '(my-define ((x 20))) ns)
(check-equal? (eval 'x ns) 20)


;===CASE 3 use introspection to prevent re-definition ====

(define-syntax-rule (my-define-v2 ([id val]))
   (begin
     (when (memq 'id (namespace-mapped-symbols ns))
           (error (format "~A already defined as ~A in this namespace\n" 
'id id)))
     (define id val)))


(eval '(my-define-v2 ((x 10))) ns)

(eval '(my-define-v2 ((x 20))) ns)
;ERROR: x already defined as 10 in this namespace