bigint __unm

Author: Sainan

docs/Runtime Environment/Bigint.md

@@ -66,6 +66,14 @@ local bigint = require "pluto:bigint"
 print(new bigint(10) % new bigint(3)) --> 1
 ```
 
+---
+### `__unm`
+Returns a new bigint with the sign bit flipped.
+```pluto
+local bigint = require "pluto:bigint"
+print(-new bigint(10)) --> -10
+```
+
 ---
 ### `bigint.pow`, `__pow`
 Performs expontentiation on two bigints. Returns a new bigint.