Add ignored file

.gitignore

@@ -4,3 +4,4 @@ node_modules/
 *.d.ts
 *.log
 yarn.lock
+!/test/types.d.ts

test/types.d.ts

@@ -0,0 +1,28 @@
+import type {Literal, Parent, PhrasingContent} from 'mdast'
+
+export {}
+
+export interface X extends Parent {
+  type: 'x'
+  children: PhrasingContent[]
+}
+
+export interface Y extends Literal {
+  type: 'y'
+}
+
+// Add nodes to content, register `data` on paragraph.
+declare module 'mdast' {
+  interface BlockContentMap {
+    x: X
+  }
+
+  interface PhrasingContentMap {
+    y: Y
+  }
+
+  interface RootContentMap {
+    x: X
+    y: Y
+  }
+}