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)
        p.chans[subj] = make(chan interface{})
        go func() {
            p.mu.Lock()
            defer p.mu.Unlock()
            for {
                p.conds[subj].Wait()
                select {
                case msg := <- p.chans[subj]:
                    for _, sub := range p.subs[subj] {
                        sub.cb(msg)
                    }
                }
            }
        }()
    }
    return &subscription, nil
}