diff --git a/prototype/UI/index.html b/prototype/UI/index.html
index 67a8a1a4b..bca32dcc4 100644
--- a/prototype/UI/index.html
+++ b/prototype/UI/index.html
@@ -211,6 +211,14 @@
});
}
}
+
+ // forall a. Diff a -> (Syntax a -> a) -> a
+ Diff.prototype.cata = function(transform) {
+ if (this.pure != null) { return this.pure; }
+ if (this.roll != null) {
+ return transform(this.roll.unwrap.map(function(diff) { return diff.cata(transform); }))
+ }
+ }