// when picking possible candidates the list is built in the order
	// of match plausibility as to save cycles in buildChains:
	//   AKID and SKID match
	//   AKID present, SKID missing / AKID missing, SKID present
	//   AKID and SKID don't match
	var matchingKeyID, oneKeyID, mismatchKeyID []potentialParent
	for _, c := range s.byName[string(cert.RawIssuer)] {
		candidate, constraint, err := s.cert(c)