Need of having observers in Participant's configuration list

[Sujay Paranjape <sp...@gmail.com>]

If I want to add observers to zookeeper cluster, why is it required to have
observers in the Quorum participant's configuration list ? We can start
observers without having them in the participant's configuration list and
leader still forwards them transactions. If I am not wrong, when a follower
/ observer joins the quorum, leader communicates to it about its leadership
and follower/ observer makes sure that majority is following the leader.
That makes it necessary to have all the participants in observers
configuration list. But is it really required for
a)  participants to know about observers ?
b) observers to know about other observers ?