docs: function transform(data, encoding, callback) {
      /* jshint validthis:true */
  
      // Ignore if killed
      /* istanbul ignore if */
      if (this._swiftTransform.killed) {
          return;
      }
  
      // Push the task
      this._swiftTransform.queue.push({ data: data, encoding: encoding }, transformDone.bind(this));
  
      // If we are not saturated, simply call more right away
      if (this._swiftTransform.queue.running() + this._swiftTransform.queue.length() < this._swiftTransform.queue.concurrency) {
          setImmediate(callback);
      // Otherwise store the callback because we will need it later
      } else {
          this._swiftTransform.callback = callback;
      }
  }