query.go
func SelectMany(subject Enumerable, toMany Unfolder) Enumerable {
	return selectManyer{
		original: subject,
		toMany:   toMany,
	}
}
func SelectMany(subject Enumerable, toMany Unfolder) Enumerable {
	return selectManyer{
		original: subject,
		toMany:   toMany,
	}
}
// SelectMany allows for unfolding of values.