fix negativity

beautiful-racket-demo/precalc-demo/grammar.rkt

@@ -6,6 +6,7 @@ fun : /"fun" var /"(" argvars /")" /"=" expr
 @expr : add-or-sub
 add-or-sub : [add-or-sub ("+" | "-")] mult-or-div
 mult-or-div : [mult-or-div ("*" | "/")] value
-@value : var | INT | app | /"(" expr /")"
+@value : var | int | app | /"(" expr /")"
+int : ["-"] INT 
 app : var /"(" [expr (/"," expr)*] /")"
 @var : ID

beautiful-racket-demo/precalc-demo/main.rkt

@@ -1,6 +1,6 @@
 #lang br/quicklang
 (require brag/support "grammar.rkt")
-(provide top fun app add-or-sub mult-or-div)
+(provide top fun app add-or-sub mult-or-div int)
 
 (module+ reader
   (provide read-syntax))
@@ -34,6 +34,10 @@
   [(_ LEFT "/" RIGHT) #'(/ LEFT RIGHT)]
   [(_ OTHER) #'OTHER])
 
+(define-macro-cases int
+  [(_ VAL) #'VAL]
+  [(_ "-" VAL) #'(- VAL)])
+
 (define-macro app #'#%app)
 
 (define (read-syntax src ip)