code: func (ns *Namespace) Group(key interface{}, items interface{}) (interface{}, error) {
  	if key == nil {
  		return nil, errors.New("nil is not a valid key to group by")
  	}
  
  	if g, ok := items.(collections.Grouper); ok {
  		return g.Group(key, items)
  	}
  
  	in := newSliceElement(items)
  
  	if g, ok := in.(collections.Grouper); ok {
  		return g.Group(key, items)
  	}
  
  	return nil, fmt.Errorf("grouping not supported for type %T %T", items, in)
  }
docs: // Group groups a set of elements by the given key.
  // This is currently only supported for Pages.