code: public final void addChildAfter(Node newChild, @Nullable Node node) {
      checkArgument(newChild.next == null, "The new child node has next siblings.");
      checkArgument(newChild.previous == null, "The new child node has previous siblings.");
      // NOTE: newChild.next remains null
      newChild.previous = newChild;
      addChildrenAfter(newChild, node);
    }
docs: Add 'newChild' after 'node'.  If 'node' is null, add it to the front of this node.