code: func (d *Driver) Exists(id string) bool {
  	rID, err := d.resolveID(id)
  	if err != nil {
  		return false
  	}
  	result, err := hcsshim.LayerExists(d.info, rID)
  	if err != nil {
  		return false
  	}
  	return result
  }
docs: // Exists returns true if the given id is registered with this driver.