code: func (v *TreeIter) Copy() (*TreeIter, error) {
  	c := C.gtk_tree_iter_copy(v.native())
  	if c == nil {
  		return nil, nilPtrErr
  	}
  	t := &TreeIter{*c}
  	runtime.SetFinalizer(t, (*TreeIter).free)
  	return t, nil
  }
docs: // Copy() is a wrapper around gtk_tree_iter_copy().