diff --git a/lib/type.js b/lib/type.js
index 9abd912..824c86e 100644
--- a/lib/type.js
+++ b/lib/type.js
@@ -66,5 +66,10 @@ module.exports = {
         length: end - start,
       });
     },
+
+    invertWithDoc: function(delta, doc) {
+      delta = new Delta(delta);
+      return delta.invert(doc);
+    }
   }
 };