diff --git a/deno/extensible.ts b/deno/extensible.ts
new file mode 100644
index 0000000..e69de29
diff --git a/deno/interfaces.ts b/deno/interfaces.ts
index 3914b1b..48dd302 100644
--- a/deno/interfaces.ts
+++ b/deno/interfaces.ts
@@ -1 +1,3 @@
-export interface Extensible {}
+type Brand<K, T> = K & { readonly __brand: T };
+
+export type Extensible = Brand<"Extensible", {}>;