code: func (p *Pages) Focus(delegate func(p Primitive)) {
  	if delegate == nil {
  		return // We cannot delegate so we cannot focus.
  	}
  	p.setFocus = delegate
  	var topItem Primitive
  	for _, page := range p.pages {
  		if page.Visible {
  			topItem = page.Item
  		}
  	}
  	if topItem != nil {
  		delegate(topItem)
  	}
  }
docs: // Focus is called by the application when the primitive receives focus.