Just want to mention that Zulip is not a Slack clone. The development of Zulip was started in August 2012[1] while Slack was released in August 2013. Folks who are interested to know more should take a look at https://zulipchat.com/why-zulip/

[1] https://github.com/zulip/zulip/commit/ea43d2e40e59cb0240ea36...