query.go
func ElementAt(iter Enumerable, n uint) interface{} {
	done := make(chan struct{})
	defer close(done)
	return iter.Enumerate(done).ElementAt(n)
}
func ElementAt(iter Enumerable, n uint) interface{} {
	done := make(chan struct{})
	defer close(done)
	return iter.Enumerate(done).ElementAt(n)
}
// ElementAt retreives an item at a particular position in an Enumerator.