CheckUnd checks whether s is correctly configured with `und` struct tag option without validating it.

ValidateUnd validates whether s is compliant to the constraint placed by `und` struct tag.

The field is allowed to be Some or Defined.

Only for elastic types.

The field is allowed to be None or Null.

The field must be nullish(None, Null, Undefined).

The field must be required(Some or Defined).

The field is allowed to be None or Undefined.

Only for elastic types.

ErrMalformedLen is an error which will be returned by ValidateUnd and CheckUnd if an input has malformed len option in `und` struct tag.

ErrMalformedLen is an error which will be returned by ValidateUnd and CheckUnd if an input has malformed values option in `und` struct tag.

ErrMultipleOption would be returned by ValidateUnd and CheckUnd if input's `und` struct tags have multiple mutually exclusive options.

ErrNotStruct would be returned by ValidateUnd and CheckUnd if input is not a struct nor a pointer to a struct.

ErrUnknownOption is an error value which will be returned by ValidateUnd and CheckUnd if an input has unknown options in `und` struct tag.

CheckerUnd wraps the CheckUnd method which is expected to be implemented on data container types like und.Und[T] and option.Option[T], etc.

ValidatorUnd wraps the ValidateUnd method.