When receiving a message via a subscription to a list of channels via multiplexing, how can I tell what channel it came from?

There are three optional parameters to the subscribe message callback: message, envelope and channel. Here is an example of how to use these three parameters.

pubnub.subscribe({
    channel: 'channel1,channel2,abc,realtime,petunia',
    message: function(msg, env, ch) {
        console.log("channel: " + ch + 
            "; envelope: " + env + 
            "; message: " + JSON.stringify(msg));
    }
});

setTimeout(function() {
    pubnub.publish({
        'channel':'channel1', 
        'message':'this is a message'})},
    2000);

setTimeout(function() {
    pubnub.publish({
        'channel':'channel2', 
        'message':'this is another message'})},
    2000);

setTimeout(function() {
    pubnub.publish({
        'channel':'petunia', 
        'message':'oh no, not again?'})},
    2000);