code: func (v *List) DataWrapper(fn func(unsafe.Pointer) interface{}) {
  	if v == nil {
  		return
  	}
  	v.dataWrap = fn
  }
docs: // 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.