code: func (authz *Authorization) FindChallenge(challengeID int64) int {
  	for i, c := range authz.Challenges {
  		if c.ID == challengeID {
  			return i
  		}
  	}
  	return -1
  }
docs: // FindChallenge will look for the given challenge inside this authorization. If
  // found, it will return the index of that challenge within the Authorization's
  // Challenges array. Otherwise it will return -1.