Fix bug in implementation

code/symbols-as-strings.sld

@@ -32,10 +32,10 @@
                    (symbol->string symbol2)))
 
     (define (symbol-prefix? prefix symbol)
-      (string-prefix? prefix symbol))
+      (string-prefix? prefix (symbol->string symbol)))
 
     (define (symbol-suffix? suffix symbol)
-      (string-suffix? suffix symbol))
+      (string-suffix? suffix (symbol->string symbol)))
 
     (define symbol-middle
       (case-lambda