Lower type bounds
trait Bird
case class AfricanSwallow() extends Bird
case class EuropeanSwallow() extends Bird
val africanSwallowList = ListNode[AfricanSwallow](AfricanSwallow(), Nil())
val birdList: Node[Bird] = africanSwallowList
birdList.prepend(EuropeanSwallow())
Declare a type to be a supertype of another type.
Lower type bounds → are a → Scala Unified types.