SUBMIT_SMRESP not received in the callback.

[Munmendu]

          In transmitter.go file  rc != nil,which eventually not calling the f(p). After changing to else if to if ,submit_smresp event is being received in the transceiver callback. Could you please help me why its being written like this? 
          seq := p.Header().Seq
    t.tx.Lock()
    rc := t.tx.inflight[seq]
    t.tx.Unlock()
    if rc != nil {
        rc <- &tx{PDU: p}
    } else if f != nil {
        f(p)
    }