How to use the @lumino/algorithm.ChainIterator function in @lumino/algorithm

iterUserWidgets(): IIterator {
      let children = map(this.children, child => child.iterUserWidgets());
      return new ChainIterator(children);
    }

iterHandles(): IIterator {
      let children = map(this.children, child => child.iterHandles());
      return chain(this.handles, new ChainIterator(children));
    }

iterTabBars(): IIterator> {
      let children = map(this.children, child => child.iterTabBars());
      return new ChainIterator>(children);
    }

iterSelectedWidgets(): IIterator {
      let children = map(this.children, child => child.iterSelectedWidgets());
      return new ChainIterator(children);
    }

iterAllWidgets(): IIterator {
      let children = map(this.children, child => child.iterAllWidgets());
      return new ChainIterator(children);
    }