code: func MinDuration(candidates ...*duration.Duration) (exp time.Duration) {
  	for _, c := range candidates {
  		if cd := google.DurationFromProto(c); cd > 0 && (exp <= 0 || cd < exp) {
  			exp = cd
  		}
  	}
  	return
  }
docs: // MinDuration selects the smallest duration that is > 0 from a set of
  // duration.Duration protobufs.
  //
  // If none of the supplied Durations are > 0, 0 will be returned.