code: function (key, task, done, ttl) {
      const queue     = this.queue;
      const semaphore = this.semaphore;
  
      if (!ttl) ttl = this.ttl;
  
  
      const runTask = (res) => new Promise((resolve) =>
        task( (...args) => resolve(Response.setData(res, args)) )
      );
  
  
      const startThenRunTask = (lock_key) =>
        queue.start(key, Response.factory(ttl))
        .then(runTask)
        .then(queue.dequeue.bind(null, key))
        .then(() => semaphore.release(key, lock_key))
        .catch((err) => {
          semaphore.release(key, lock_key);
          throw err;
        })
      ;
  
  
      const startIfNotWaiting = () =>
        Promise.all([
          queue.isWaiting(key)
        , semaphore.lock(key)
        ])
        .then(([ is_waiting, lock_key ]) =>
          (!is_waiting && lock_key) ? startThenRunTask(lock_key) : null
        )
  
      return queue
        .add(key, done)
        .then(startIfNotWaiting)
      ;
    }
docs: Queue up the given task using the given key as an identifier. @param {string} key @param {Function} task @param {Function} done @param {number} ttl - the time to live for the queue cache. @return {Queue}