anchor.diff:    /// can use this information to determine the shape of the semantics tree.
     ///
     /// See also:
  +  ///
     ///  * [RenderSemanticsGestureHandler.excludeFromScrolling] for an example of
     ///    how tags are used.
     void addTagForChildren(SemanticsTag tag) {
  
positive.code:   /// registered.
    ///
    /// See also:
    ///  * [addAction] to add an action.
    VoidCallback getActionHandler(SemanticsAction action) => _actions[action];
  
  
negative.code:   /// decedent nodes are merged into this node.
    ///
    /// See also:
    ///
    ///  * [isMergedIntoParent]
    ///  * [mergeAllDescendantsIntoThisNode]
    bool get isPartOfNodeMerging => mergeAllDescendantsIntoThisNode || isMergedIntoParent;
  