query.go
func Where(original Enumerable, p Predicate) Enumerable {
	return wherer{
		original: original,
		filter:   p,
	}
}
func Where(original Enumerable, p Predicate) Enumerable {
	return wherer{
		original: original,
		filter:   p,
	}
}
// Where creates a reusable means of filtering a stream.