Kotlin provides the ability to extend a class with new functionality without 
having to inherit from the class or use design patterns such as Decorator. 
This is done via special declarations called extensions.

fun MutableList.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}