code: func (p *Prependable) Prepend(size int) []byte {
  	if size > p.usedIdx {
  		return nil
  	}
  
  	p.usedIdx -= size
  	return p.View()[:size:size]
  }
docs: // Prepend reserves the requested space in front of the buffer, returning a
  // slice that represents the reserved space.