code: func (authz *Authorization) FindChallengeByStringID(id string) int {
  	for i, c := range authz.Challenges {
  		if c.StringID() == id {
  			return i
  		}
  	}
  	return -1
  }
docs: // FindChallengeByStringID will look for a challenge matching the given ID inside
  // this authorization. If found, it will return the index of that challenge within
  // the Authorization's Challenges array. Otherwise it will return -1.