data class Node<T : Comparable<T>>(
   val key: T,
   val left: Node<T>? = null,
   val right: Node<T>? = null
)

fun <T : Comparable<T>> minIterative(root: Node<T>): Node<T> {
   var curr = root
   while (curr.left != null) {
      curr = curr.left // Smart cast to Node<T> is impossible, because 'curr.left' is a complex expression
   }
   return curr
}