 class1 = MyClass(t: 10, u: "You")
 
 let class2 = MyClass(t: 12, u: "World")
-class2.u = "Planet" // No error
+
+class2.u = "Planet" // No error, Pay attention to here
+// Although class is let, you still can modify u
+// Reason: Because in Swift, class is reference type.
+
 //class2 = MyClass(t: 11, u: "Geoid") Error: class2 is an immutable reference
 
 
 var array1 = [1,2,3,4]
 array1.append(5)
+array1
 array1[0] = 27
 array1
 array1 = [3,2]

let struct2 = MyStruct(t: 12, u: "World")
//struct2.u = "Planet" // Error: struct2 is immutable
//struct2 = MyStruct(t: 10, u: "Defeat") // Error: struct2 is an immutable ref

class MyClass {
  let t = 12

  // Do something
}

// Top-tip: Casting array is still easy
func someArrayFunc(parameter: [AnyObject]!) {
  let newArray = parameter as [NSString]
  // Do something
