func activeEmails(emails es)(emails){
    length := len(es)
    switch length{
        case 0:
            return nil
        case 1:
            return emails.Active ? emails:nil
        default:
            fchan := make(chan emails, 2)
            f := func (emails slice){
                output := []
                for i : range slice {
                    if (slice[i].Active){
                        output = append(output, slice[i])
                    }
                }
                fchan <- output
            }
            half := length/2
            go f(es[0:half])
            go f(es[half:length])
            f1 <- fchan
            f2 <- fchan
            close(fchan)
            return append(f1, f2...)
    }
}