How do you use a nil channel in select to dynamically enable or disable cases?

// Scenario: drain primary channel; only then activate secondary
func processOrdered(primary, secondary <-chan string) {
    activePrimary   := primary
    activeSecondary := (<-chan string)(nil) // disabled initially

    for activePrimary != nil || activeSecondary != nil {
        select {
        case v, ok := <-activePrimary:
            if !ok {
                activePrimary   = nil      // disable primary case
                activeSecondary = secondary // enable secondary case
            } else {
                fmt.Println("primary:", v)
            }
        case v, ok := <-activeSecondary:
            if !ok {
                activeSecondary = nil
            } else {
                fmt.Println("secondary:", v)
            }
        }
    }
}

// Another use: conditional send — nil channel means 'skip this send'
func maybeSend(notify chan<- string, msg string, doSend bool) {
    var ch chan<- string
    if doSend { ch = notify } // ch is nil if !doSend
    select {
    case ch <- msg: // disabled when ch is nil — never fires
    default:
    }
}

// Key property:
// A nil channel in select is excluded from consideration entirely.
// It does not block; it does not fire. It simply doesn't exist.