Grabbed dndbeyond's source code

```
~/go/bin/sourcemapper -output ddb -jsurl https://media.dndbeyond.com/character-app/static/js/main.90aa78c5.js
```

ddb_main/packages/rules-engine/es/engine/Background/derivers.js

@@ -0,0 +1,8 @@
+import { getEntityTypeId, getId } from './accessors';
+/**
+ *
+ * @param background
+ */
+export function deriveUniqueKey(background) {
+    return [getId(background), getEntityTypeId(background)].join('-');
+}