code: func (r *IsRevokedCertRPC) IsRevokedCert(c context.Context, req *admin.IsRevokedCertRequest) (*admin.IsRevokedCertResponse, error) {
  	sn := big.Int{}
  	if _, ok := sn.SetString(req.Sn, 0); !ok {
  		return nil, status.Errorf(codes.InvalidArgument, "can't parse 'sn'")
  	}
  
  	checker, err := GetCertChecker(c, req.Ca)
  	if err != nil {
  		if details, ok := err.(Error); ok && details.Reason == NoSuchCA {
  			return nil, status.Errorf(codes.NotFound, "no such CA: %q", req.Ca)
  		}
  		return nil, status.Errorf(codes.Internal, "failed to check %q CRL - %s", req.Ca, err)
  	}
  
  	revoked, err := checker.CRL.IsRevokedSN(c, &sn)
  	if err != nil {
  		return nil, status.Errorf(codes.Internal, "failed to check %q CRL - %s", req.Ca, err)
  	}
  
  	return &admin.IsRevokedCertResponse{Revoked: revoked}, nil
  }
docs: // IsRevokedCert says whether a certificate serial number is in the CRL.