Restrict variables to avoid overlap with keywords.

Author: dvanhorn

www/notes/fraud/syntax.rkt

@@ -5,7 +5,7 @@
 ;; Is x a well-formed expression?
 (define (expr? x)
   (match x
-    [(? symbol?) #t]
+    [(? symbol?) (not (memq x '(if let add1 sub1)))]
     [(? integer? i) #t]
     [`(add1 ,x) (expr? x)]
     [`(sub1 ,x) (expr? x)]