interface Set<out E> : Collection<E>

abstract fun contains(element: E): Boolean
abstract fun containsAll(elements: Collection<E>): Boolean

class StringSet : Set<String> {
    override val size = 2
    override fun contains(element: String): Boolean {
        println("--- StringSet.contains($element)")
        return element == "Hallo" || element == "World"
    }

    override fun containsAll(elements: Collection<String>) : Boolean =
        elements.all({it -> contains(it)})
    override fun isEmpty() = false
    override fun iterator() = listOf("Hallo", "World").iterator()

}

fun main() {
    val sset : Set<String> = StringSet()
    println(sset.contains("Hallo"))
    println(sset.contains("xxx"))
    //// compiler error:
    // println(set.contains(5))

    val aset : Set<Any> = sset
    println(aset.contains("Hallo"))
    println(aset.contains("xxx"))
    // this compiles (and returns false), but the method is not actually called
    println(aset.contains(5))
}

abstract fun contains(element: @UnsafeVariance E): Boolean
abstract fun containsAll(elements: Collection<@UnsafeVariance E>): Boolean