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diff --git a/ui/accessibility/ax_tree_update.h b/ui/accessibility/ax_tree_update.h
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+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
+#define UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
+
+#include <vector>
+
+#include "ui/accessibility/ax_node_data.h"
+
+namespace ui {
+
+// An AXTreeUpdate is a serialized representation of an atomic change
+// to an AXTree. The sender and receiver must be in sync; the update
+// is only meant to bring the tree from a specific previous state into
+// its next state. Trying to apply it to the wrong tree should immediately
+// die with a fatal assertion. An AXTreeUpdate is just an ordered vector
+// of AXNodeData structures to be applied to the tree in order.
+//
+// Suppose that the next AXNodeData to be applied is |node|. The following
+// invariants must hold:
+// 1. Either |node.id| is already in the tree, or else |node| is the new
+//        root of the tree and |node.role| == WebAXRoleRootWebArea.
+// 2. Every child id in |node.child_ids| must either be already a child
+//        of this node, or a new id not previously in the tree. It is not
+//        allowed to "reparent" a child to this node without first removing
+//        that child from its previous parent.
+// 3. When a new id appears in |node.child_ids|, the tree should create a
+//        new uninitialized placeholder node for it immediately. That
+//        placeholder must be updated within the same AXTreeUpdate, otherwise
+//        it's a fatal error. This guarantees the tree is always complete
+//        before or after an AXTreeUpdate.
+struct AX_EXPORT AXTreeUpdate {
+  AXTreeUpdate();
+  ~AXTreeUpdate();
+
+  std::vector<AXNodeData> nodes;
+
+  // TODO(dmazzoni): location changes
+};
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_TREE_UPDATE_H_