code: function(client, subscriptions, channel, callback, done_callback) {
      if (subscriptions[channel]===undefined)
          return;
      subscriptions[channel].splice(subscriptions[channel].indexOf(callback), 1);
      if (subscriptions[channel].length===0) {
          delete subscriptions[channel];
          client.unsubscribe(channel, function() {
              // If there is a done_callback defined, execute it
              if (done_callback!==undefined) done_callback();
          });
      }
  }
docs: Helper function that unsubscribes from a channel in node