var a = [1],
  b = [2],
  c = [3];
var wset = new WeakSet([a, b, a]);
wset.add(c).add(b).add(c);
log(wset.has(b)); // => true
log(wset.has([2])); // => false
wset["delete"](b);
log(wset.has(b)); // => false