code: function LookupList(id, capacity) {
      if (!(id instanceof Id))
          throw new Error('invalid id or selfId');
      if (!(typeof capacity === 'number' && capacity > 0))
          throw new Error('invalid capacity');
      this._id = id;
      this._capacity = capacity;
      this._slots = [];
  }
docs: Store contacts in a sorted manner, from closest to fartest from the specified `id`.