Set<V>
using a Map<V, Bool>
implementation.
abstract Set<V> {
final length: Int;
function new(?values: Iterable<V>);
function add(value: V): Void;
function remove(value: V): Bool;
function exists(value: V): Bool;
function iterator(): Iterator<V>;
function copy(): Set<V>;
function toArray(): Array<V>;
function toString(): String;
function clear(): Void;
}