diff --git a/package.json b/package.json
--- a/package.json
+++ b/package.json
@@ -23,6 +23,7 @@
     "d3": "^7.8.4",
     "debug": "^4.3.4",
     "fpsmeter": "^0.3.1",
+    "immutable": "^4.3.0",
     "knockout": "^3.5.1",
     "lit": "^2.7.4",
     "n3": "^1.16.4",