add `push!` and `pop!`

beautiful-racket-lib/br/list.rkt

@@ -4,3 +4,11 @@
          
 (define-macro (values->list EXPR)
   #'(call-with-values (λ () EXPR) list))
+
+(define-macro (push! ID VAL)
+  #'(set! ID (cons VAL ID)))
+
+(define-macro (pop! ID)
+  #'(let ([x (car ID)])
+    (set! ID (cdr ID))
+    x))