Testing with the failure.

tests/SimpleTest.php

@@ -13,6 +13,14 @@ class SimpleTest extends TestCase
         $this->assertEquals(5, $result);
     }
 
+    public function testSubtractFail()
+    {
+        $simple = new Simple();
+        $result = $simple->subtract(4, 2);
+        // 2 is the right result, therefore I chose not 2.
+        $this->assertEquals(1, $result);
+    }
+
     public function testSubtract()
     {
         $simple = new Simple();