code: function zincrby(key, increment, member, req) {
    var val = this.getKey(key, req);
    if(val === undefined) {
      val = new SortedSet();
      this.setKey(key, val, undefined, undefined, undefined, req);
    }
    return val.zincrby(increment, member);
  }
docs: Increments the score of member in the sorted set stored
  at key by increment.