I want to make a `power`

function such that it can only accept Long or Int or BigInteger as argument. I tried doing it as:

```
fun pow(a: Number, b: Int): Number {
if (b == 0) return 1
if (b == 1) return a
return when (a) {
is Long -> {
if (b % 2 == 0) pow(a * a, b / 2)
else a * pow(a * a, b / 2) as Long
}
is Int -> {
if (b % 2 == 0) pow(a * a, b / 2)
else a * pow(a * a, b / 2) as Int
}
is BigInteger -> {
if (b % 2 == 0) pow(a * a, b / 2)
else a * pow(a * a, b / 2) as BigInteger
}
else -> throw IllegalArgumentException("only Long, Int, BigInteger are supported")
}
}
```

but this does not fail at compile time for decimal argument. Is there a better way to do this?