code: func Cost(hashedPassword []byte) (int, error) {
  	p, err := newFromHash(hashedPassword)
  	if err != nil {
  		return 0, err
  	}
  	return p.cost, nil
  }
docs: // Cost returns the hashing cost used to create the given hashed
  // password. When, in the future, the hashing cost of a password system needs
  // to be increased in order to adjust for greater computational power, this
  // function allows one to establish which passwords need to be updated.