code: func (p *pageState) Group(key interface{}, in interface{}) (interface{}, error) {
  	pages, err := page.ToPages(in)
  	if err != nil {
  		return nil, err
  	}
  	return page.PageGroup{Key: key, Pages: pages}, nil
  }
docs: // collections.Grouper  implementations below
  // Group creates a PageGroup from a key and a Pages object
  // This method is not meant for external use. It got its non-typed arguments to satisfy
  // a very generic interface in the tpl package.