code: func wildcardOverlap(dnsNames []string) error {
  	nameMap := make(map[string]bool, len(dnsNames))
  	for _, v := range dnsNames {
  		nameMap[v] = true
  	}
  	for name := range nameMap {
  		if name[0] == '*' {
  			continue
  		}
  		labels := strings.Split(name, ".")
  		labels[0] = "*"
  		if nameMap[strings.Join(labels, ".")] {
  			return berrors.MalformedError(
  				"Domain name %q is redundant with a wildcard domain in the same request. Remove one or the other from the certificate request.", name)
  		}
  	}
  	return nil
  }
docs: // wildcardOverlap takes a slice of domain names and returns an error if any of
  // them is a non-wildcard FQDN that overlaps with a wildcard domain in the map.