Suggestion for a new code example. PR637

doc/handbook/Interfaces.md

@@ -502,20 +502,21 @@ interface SelectableControl extends Control {
     select(): void;
 }
 
-class Button extends Control {
+class Button extends Control implements SelectableControl {
     select() { }
 }
 
 class TextBox extends Control {
-    select() { }
+
 }
 
-class Image {
+// Error: Property 'state' is missing in type 'Image'.
+class Image implements SelectableControl {
     select() { }
 }
 
 class Location {
-    select() { }
+
 }
 ```