Use the following ASN.1 definition suggested by Nicholas Williams
DEFINITION EXPLICIT TAGS ::= BEGIN
IMPORTS Certificate, CertificateList FROM PKIX1Explicit93
CertificateOrCRL ::= CHOICE { cert [0] Certificate, crl [1] CertificateList }
CertificateBundle ::= SEQUENCE OF CertificateOrCRL END
--Paul Hoffman, Director --VPN Consortium