To declare an extension function, prefix its name with a receiver 
type, which refers to the type being extended. The following adds a 
swap function to MutableList:

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