code: function sdiffstore(destination /* key-1, key-N, req*/) {
    var args = slice.call(arguments, 1)
      , req = typeof args[args.length - 1] === 'object'
        ? args[args.length - 1] : null;
    var list = this.sdiff.apply(this, args);
    if(list.length) {
      this.setKey(
        destination, new Set(list), undefined, undefined, undefined, req);
    }
    return list.length;
  }
docs: Stores the members of the set resulting from the difference
  between the first set and successive sets.