Index: node_modules/d3-transition/src/transition/merge.js
===================================================================
--- node_modules/d3-transition/src/transition/merge.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-transition/src/transition/merge.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,19 @@
+import {Transition} from "./index.js";
+
+export default function(transition) {
+  if (transition._id !== this._id) throw new Error;
+
+  for (var groups0 = this._groups, groups1 = transition._groups, m0 = groups0.length, m1 = groups1.length, m = Math.min(m0, m1), merges = new Array(m0), j = 0; j < m; ++j) {
+    for (var group0 = groups0[j], group1 = groups1[j], n = group0.length, merge = merges[j] = new Array(n), node, i = 0; i < n; ++i) {
+      if (node = group0[i] || group1[i]) {
+        merge[i] = node;
+      }
+    }
+  }
+
+  for (; j < m0; ++j) {
+    merges[j] = groups0[j];
+  }
+
+  return new Transition(merges, this._parents, this._name, this._id);
+}