code: func (r *reader) SetReadahead(readahead int64) {
  	r.mu.Lock()
  	r.readahead = readahead
  	r.mu.Unlock()
  	r.t.cl.lock()
  	defer r.t.cl.unlock()
  	r.posChanged()
  }
docs: // Configure the number of bytes ahead of a read that should also be
  // prioritized in preparation for further reads.