code: func (v *TreeModel) IterNChildren(iter *TreeIter) int {
  	var cIter *C.GtkTreeIter
  	if iter != nil {
  		cIter = iter.native()
  	}
  	c := C.gtk_tree_model_iter_n_children(v.native(), cIter)
  	return int(c)
  }
docs: // IterNChildren is a wrapper around gtk_tree_model_iter_n_children().