Haha, yeah, they don't have a decentralized event store, where you can mix-and-match-and-merge things like that.

Your meme is 🎯 

 Actually Reddit permits crossposting:
https://support.reddithelp.com/hc/en-us/articles/4835584113684-What-is-Crossposting 

 That's not what I mean though.
I mean 1 post simultaneously in r/community1,  r/community2, r/community3, ... 

 Yes, this was just to point out that there is a similar perception of the necessity on Reddit. I understand you are thinking of a more complex interaction, where the user can join the conversation without leaving their group. 
Maybe this is difficult because a thread removed from the community context can be misleading to someone who joins from outside, so the conversation becomes more complex. 

 Hmm, yeah maybe. I see. 
Do you know if this feature is used often? 
It doesn't seem ideal for the creator since it's extra work during publishing and it's hard to keep up with the discussion in several separated communities. 

 Sorry I don't know, I don't use reddit so much; I know that several communities block them to avoid easy spam.
Btw, crossposting on Reddit is like a repost, so we already have it in its basic form.
 

 I meant ANY app in the universe. 
I know this is technically possible on Nostr, it's built for that, that's why I'm asking 😉   

 well the answer is "the client has to support it" 

 You could just turn that tag into an array and list as many, as you like. 🤷‍♀️ 

 tags are arrays already silly

nothing stopping you from putting more than one with the same first field (the type field) just the client may only read one of them and that may be random 

 Wait, am I? 
How would Fedimint solve this better than Nostr?  

 Greeeeaaat question!
Been thinking about this problem a lot and what I'd prefer myself is: 

1. You can only post chat messages to 1 group
2. But you can publish posts, articles, wikis, docs, books, git problems, etc... in multiple groups at once 
3. You can determine per group what you get notified about on your home screen
https://cdn.satellite.earth/62e3437aba0b4e51d8231807344ef056079a6a845a91594688910cde52e9b6ee.png
Since I'm giving every category an emoji-icon in the group's menu, I can reuse these as a compact way of displaying the telegram-like counters in the home screen. 
https://cdn.satellite.earth/f0b0f7d32302f4772c3128a4d97ab5897d54cf50a66499e2f669a36a6d0d879e.png

4. The home screen let's you open feeds by content category (posts, articles, ...).
https://cdn.satellite.earth/3d53d9393a6369940a33938ab9880971a078447153a56ee0c0f10860bf6e82a3.mov 

 5. MOST IMPORTANTLY: the notification count on other chats (for post f.e.) would go down for every post you've already read somewhere else. 

 Ok this one is solving the notification counter! But it means you'd have to re-order the feed when I open the community so that stuff I already read in other places falls down, and stuff I haven't read pops up, even if it's older. Right? 

 Yes exactly! Displaying the NEW stuff first within the community and letting the ALREADY SEEN / OLD stuff fall down (but not hiding it). 

 Suppose community has so many new posts (made worse by being able to post to many at once) that it's feed becomes too big to sift through. The more popular the community is, the more likely it's feed will be huge. Shall we need an algo then? 

 What's chat message vs post? Chat message has 'root'? 

 https://github.com/nostr-protocol/nips/blob/master/29.md
It's the two examples given in the NIP so far.
(though, I don't really get yet why it's not just kind 1's for post for example)
But as far as I'm concenrned it's a different use case.

Chat = "Gm frens", chronological chaotic feed, fun and giggles
Post / Thread = "Here's my thought I'd like to have an more depth conversation around) 

 Thanks, that nip is enormous, need to think it through  

 I love the icons and counters and overall the attempt to integrate different kinds of stuff under one "destination"!