descendantTopicExists = function(topic) {
                var m;
                for ( m in messages ){
                    if ( Object.prototype.hasOwnProperty.call(messages, m) && m.indexOf(topic) === 0 ){
                        // a descendant of the topic exists:
                        return true;
                    }
                }

                return false;
            }
Removes subscriptions

- When passed a token, removes a specific subscription.

- When passed a function, removes all subscriptions for that function

- When passed a topic, removes all subscriptions for that topic (hierarchy)
@function
@public
@alias subscribeOnce
@param { String | Function } value A token, function or topic to unsubscribe from
@example // Unsubscribing with a token
var token = PubSub.subscribe('mytopic', myFunc);
PubSub.unsubscribe(token);
@example // Unsubscribing with a function
PubSub.unsubscribe(myFunc);
@example // Unsubscribing from a topic
PubSub.unsubscribe('mytopic');