diff --git a/string/doc/usage.xml b/string/doc/usage.xml
index ca100e0..528babe 100644
--- a/string/doc/usage.xml
+++ b/string/doc/usage.xml
@@ -130,7 +130,7 @@
     string str1("command.com");
     cout 
         &lt;&lt; str1
-        &lt;&lt; is_executable("command.com")? "is": "is not" 
+        &lt;&lt; (is_executable("command.com")? "is": "is not") 
         &lt;&lt; "an executable" 
         &lt;&lt; endl; // prints "command.com is an executable"
     
@@ -138,7 +138,7 @@
     char text1[]="hello world!";
     cout 
         &lt;&lt; text1 
-        &lt;&lt; all( text1, is_lower() )? "is": "is not"
+        &lt;&lt; (all( text1, is_lower() )? "is": "is not")
         &lt;&lt; " written in the lower case" 
         &lt;&lt; endl; // prints "hello world! is written in the lower case"
         </programlisting>
