code: func SizeGroupNew(mode SizeGroupMode) (*SizeGroup, error) {
  	c := C.gtk_size_group_new(C.GtkSizeGroupMode(mode))
  	if c == nil {
  		return nil, nilPtrErr
  	}
  	return wrapSizeGroup(glib.Take(unsafe.Pointer(c))), nil
  }
docs: // SizeGroupNew is a wrapper around gtk_size_group_new().