code: func UserStructLevelValidation(sl validator.StructLevel) {
  
  	user := sl.Current().Interface().(User)
  
  	if len(user.FirstName) == 0 && len(user.LastName) == 0 {
  		sl.ReportError(user.FirstName, "FirstName", "fname", "fnameorlname", "")
  		sl.ReportError(user.LastName, "LastName", "lname", "fnameorlname", "")
  	}
  
  	// plus can do more, even with different tag than "fnameorlname"
  }
docs: // UserStructLevelValidation contains custom struct level validations that don't always
  // make sense at the field validation level. For Example this function validates that either
  // FirstName or LastName exist; could have done that with a custom field validation but then
  // would have had to add it to both fields duplicating the logic + overhead, this way it's
  // only validated once.
  //
  // NOTE: you may ask why wouldn't I just do this outside of validator, because doing this way
  // hooks right into validator and you can combine with validation tags and still have a
  // common error output format.