func (p *MyPubSub) Subscribe(subj string, cb MsgHandler) (Subscription, error) {
    p.mu.Lock()
    defer p.mu.Unlock()
    subscription := MySubscription{pubSub: p, subj: subj, cb: cb}
    p.subs[subj] = append(p.subs[subj], &subscription)
    if _, ok := p.conds[subj]; !ok {
        p.conds[subj] = sync.NewCond(&p.mu)
    }
    go func() {
        p.conds[subj].Wait()

    }()
    return &subscription, nil
}