How to do it...

1. Create a new file, Extensions.kt, to store the extension function definition. 
2. Implement the swap() function inside:

fun <T> Array<T>.swap(a: T, b: T) {
    val positionA = indexOf(a)
    val positionB = indexOf(b)

    if (positionA < 0 || positionB < 0) {
        throw IllegalArgumentException("Given param doesn't belong
        to the array")
    }

    val tmp = this[positionA]
    this[positionA] = this[positionB]
    this[positionB] = tmp
}