NIP-91: AND operator for filters

91.md

@@ -0,0 +1,20 @@
+# NIP-91: AND Operator in Filters
+
+Enable `AND` within a single tag filter by using an `&` modifier in filters for indexable tags.
+
+```
+filters: {
+  "kinds": [1],
+  "&t": ["meme", "cat"],
+  "#t": ["meme", "cat", "black", "white"]
+}
+// returns kind `1` events with `t` tags that have both "meme" and "cat" that have the tag "black" or "white" by NIP-91 relays, whereas relays without NIP-91 return entire result for local intersection.
+```
+
+## Rules
+
+- `AND` **MUST** take precedence over `OR` by relays
+- Any tag value used in `AND` **SHOULD** be ignored in `OR` by relays supporting NIP-91 
+- Tag values used in `AND` by libraries and clients **MUST** include standard `OR` tags [`#`] for compatibility with relays that do not support NIP-91.
+- libraries and clients `SHOULD` run `&` post-filters after recieving results from relays; already standard practice.  
+

README.md

@@ -82,6 +82,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 - [NIP-84: Highlights](84.md)
 - [NIP-89: Recommended Application Handlers](89.md)
 - [NIP-90: Data Vending Machines](90.md)
+- [NIP-91: AND filters](91.md)
 - [NIP-92: Media Attachments](92.md)
 - [NIP-94: File Metadata](94.md)
 - [NIP-96: HTTP File Storage Integration](96.md)
@@ -201,6 +202,17 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 [lnpub]: https://github.com/shocknet/Lightning.Pub/blob/master/proto/autogenerated/client.md
 [joinstr]: https://gitlab.com/1440000bytes/joinstr/-/blob/main/NIP.md
 
+## Filters 
+
+| kind            | description                | NIP                      |
+| --------------- | -------------------------- | ------------------------ |
+| `authors`       | by pubkey                  | [01](01.md)              |
+| `ids`           | by ids                     | [01](01.md)              |
+| `kinds`         | by kinds                   | [01](01.md)              |
+| `#{}`           | by indexable tag (OR)      | [01](01.md)              |
+| `&{}`           | by indexable tag (AND)     | [91](91.md)              |
+| `search`        | by search times            | [50](50.md)              |
+
 ## Message types
 
 ### Client to Relay