code: func (authz *Authorization) SolvedBy() string {
  	if len(authz.Challenges) == 0 {
  		return ""
  	}
  	for _, chal := range authz.Challenges {
  		if chal.Status == StatusValid {
  			return chal.Type
  		}
  	}
  	return ""
  }
docs: // SolvedBy will look through the Authorizations challenges, returning the type
  // of the *first* challenge it finds with Status: valid, or "" if no challenge
  // is valid.