add functions

html.rkt

@@ -17,13 +17,21 @@
 
 (define tags (append block-tags inline-tags))
 
+(define/contract (entity->string e)
+  (symbol? . -> . string?)
+  (format "~a" (integer->char (entity->integer e))))
+
+(define/contract (string->entity str)
+  (string? . -> . symbol?)
+  (integer->entity (char->integer (car (string->list str)))))
+
 (define/contract (entity->integer e)
   (symbol? . -> . integer?)
   (hash-ref entities e))
 
-(define/contract (integer->entity c)
+(define/contract (integer->entity int)
   (integer? . -> . symbol?)
-  (hash-ref chars c))
+  (hash-ref chars int))
 
 (define entities '#hash((quot . 34)
                         (amp . 38)