mention .defined, and that type objects are undefined

src/classes-and-objects.pod

@@ -71,6 +71,8 @@ Which package?
 =end for
 
 X<type object>
+X<defined>
+X<.defined>
 
 Declaring a class creates a I<type object>, which by default gets installed
 into the package (just like a variable declared with C<our> scope).  This type
@@ -80,6 +82,21 @@ Perl 6 built-in classes.  The example uses the class name C<Task> so that other
 code can refer to it later, such as to create class instances by calling the
 C<new> method.
 
+Type objects are I<undefined>, in the sense that they return C<False> if you
+call the C<.defined> method on them. You can use this method to find out if a
+given object is a type object or not:
+
+=begin programlisting
+
+    my $obj = Int;
+    if $obj.defined {
+        say "Type object";
+    } else {
+        say "Ordinary, defined object";
+    }
+
+=end programlisting
+
 =head1 I can has state?
 
 X<attributes>