add `remove-blank-lines` for tutorial

br/main.rkt

@@ -15,5 +15,11 @@
 
 ;; todo: activate at-exp reader by default
 
+(define (remove-blank-lines strs)
+  (filter (λ(str) (regexp-match #px"\\S" str)) strs))
+
+(provide remove-blank-lines)
+
+
 (module reader syntax/module-reader
   #:language 'br)