ber.go
func UnmarshalWithParams(b []byte, val interface{}, params string) (rest []byte, err error) {
	v := reflect.ValueOf(val).Elem()
	offset, err := parseField(v, b, 0, parseFieldParameters(params))
	if err != nil {
		return nil, err
	}
	return b[offset:], nil
}
func UnmarshalWithParams(b []byte, val interface{}, params string) (rest []byte, err error) {
	v := reflect.ValueOf(val).Elem()
	offset, err := parseField(v, b, 0, parseFieldParameters(params))
	if err != nil {
		return nil, err
	}
	return b[offset:], nil
}
// UnmarshalWithParams allows field parameters to be specified for the
// top-level element. The form of the params is the same as the field tags.