func (v *List) DataWrapper(fn func(unsafe.Pointer) interface{}) {
	if v == nil {
		return
	}
	v.dataWrap = fn
}
func (v *List) DataWrapper(fn func(unsafe.Pointer) interface{}) {
	if v == nil {
		return
	}
	v.dataWrap = fn
}
// DataWapper sets wrap functions, which is called during NthDataWrapped()
// and DataWrapped(). It's used to cast raw C data into appropriate
// Go structures and types every time that data is retreived.