package foo import java.util.* fun ArrayList.plus(other: Collection): List { val c = ArrayList() c.addAll(this) c.addAll(other) return c } fun box(): Boolean { var v1 = ArrayList() v1.add("foo") val v2 = ArrayList() v2.add("bar") val v = v1 + v2 return (v.size() == 2 && v[0] == "foo" && v[1] == "bar") }