code: func (q *queue) RetrieveHeaders() ([]*types.Header, int) {
  	q.lock.Lock()
  	defer q.lock.Unlock()
  
  	headers, proced := q.headerResults, q.headerProced
  	q.headerResults, q.headerProced = nil, 0
  
  	return headers, proced
  }
docs: // RetrieveHeaders retrieves the header chain assemble based on the scheduled
  // skeleton.