/// <reference path="./interfaces/puttable.ts" />

namespace MainApp {
    /**
     * Square
     */
    export class Square {
        constructor(public piece:Puttable = null) {
        }
    }
}

namespace MainApp {
    /**
     * Square
     */
    export class Square {
        constructor() {
        }
    }
}

--- 
+++ 
@@ -1,9 +1,11 @@
+/// <reference path="./interfaces/puttable.ts" />
+
 namespace MainApp {
     /**
      * Square
      */
     export class Square {
-        constructor() {
+        constructor(public piece:Puttable = null) {
         }
     }
 }
Add piece property to Square
