How to use the asn1crypto.core.Choice function in asn1crypto

class SeqChoice(core.Choice):
    _alternatives = [
        ('one', CopySeq, {'explicit': 0}),
        ('two', CopySeq, {'implicit': 1}),
    ]


class SeqChoiceOldApi(core.Choice):
    _alternatives = [
        ('one', CopySeq, {'tag_type': 'explicit', 'tag': 0}),
        ('two', CopySeq, {'tag_type': 'implicit', 'tag': 1}),
    ]


class ChoiceChoice(core.Choice):
    _alternatives = [
        ('num', NumChoice, {'explicit': 0}),
        ('seq', SeqChoice, {'explicit': 1}),
    ]


class CCSeq(core.Sequence):
    _fields = [
        ('cc', ChoiceChoice)
    ]


class ExplicitField(core.Sequence):
    _fields = [
        ('field', NumChoice, {'tag_type': 'explicit', 'tag': 0}),
    ]

class AnyMetadata(ac.Choice):
  _alternatives = [
      ('root', RootMetadata),
      ('timestamp', TimestampMetadata),
      ('snapshot', SnapshotMetadata),
      ('targets', TargetsMetadata)]

class AnyEnvelope(ac.Choice):
  _alternatives = [
      ('root', RootEnvelope),
      ('timestamp', TimestampEnvelope),
      ('snapshot', SnapshotEnvelope),
      ('targets', TargetsEnvelope)]

class SomeType(ac.Choice):
  _alternatives = [
      ('int', ac.Integer),
      ('oct', ac.OctetString)]


# # Or we could define the following, instead of the above four Envelope classes.
#
# class AnyMetadata(ac.Choice):
#   _alternatives = [
#       ('root', RootMetadata),
#       ('timestamp', TimestampMetadata),
#       ('snapshot', SnapshotMetadata),
#       ('targets', TargetsMetadata)]
#
# class SignableEnvelope(ac.Sequence):
#   _fields = [

:return:
        An object of the type spec, or if not specified, a child of Asn1Value
    """

    if header is None:
        return VOID

    # If an explicit specification was passed in, make sure it matches
    if spec is not None:
        if spec_params:
            value = spec(contents=contents, **spec_params)
        else:
            value = spec(contents=contents)

        is_choice = isinstance(value, Choice)

        if spec == Any:
            pass

        elif value.tag_type == 'explicit':
            if class_ != value.explicit_class:
                raise ValueError(unwrap(
                    '''
                    Error parsing %s - explicitly-tagged class should have been
                    %s, but %s was found
                    ''',
                    type_name(value),
                    CLASS_NUM_TO_NAME_MAP.get(value.explicit_class),
                    CLASS_NUM_TO_NAME_MAP.get(class_, class_)
                ))
            if method != 1:

# allows Any
        return ContentInfo

    _spec_callbacks = {
        'encap_content_info': _encap_content_info_spec
    }


class OriginatorInfo(Sequence):
    _fields = [
        ('certs', CertificateSet, {'implicit': 0, 'optional': True}),
        ('crls', RevocationInfoChoices, {'implicit': 1, 'optional': True}),
    ]


class RecipientIdentifier(Choice):
    _alternatives = [
        ('issuer_and_serial_number', IssuerAndSerialNumber),
        ('subject_key_identifier', OctetString, {'implicit': 0}),
    ]


class KeyEncryptionAlgorithmId(ObjectIdentifier):
    _map = {
        '1.2.840.113549.1.1.1': 'rsaes_pkcs1v15',
        '1.2.840.113549.1.1.7': 'rsaes_oaep',
        '2.16.840.1.101.3.4.1.5': 'aes128_wrap',
        '2.16.840.1.101.3.4.1.8': 'aes128_wrap_pad',
        '2.16.840.1.101.3.4.1.25': 'aes192_wrap',
        '2.16.840.1.101.3.4.1.28': 'aes192_wrap_pad',
        '2.16.840.1.101.3.4.1.45': 'aes256_wrap',
        '2.16.840.1.101.3.4.1.48': 'aes256_wrap_pad',

class IssuerSerial(Sequence):
    _fields = [
        ('issuer', GeneralNames),
        ('serial', Integer),
        ('issuer_uid', OctetBitString, {'optional': True}),
    ]


class AttCertVersion(Integer):
    _map = {
        0: 'v1',
        1: 'v2',
    }


class AttCertSubject(Choice):
    _alternatives = [
        ('base_certificate_id', IssuerSerial, {'explicit': 0}),
        ('subject_name', GeneralNames, {'explicit': 1}),
    ]


class AttCertValidityPeriod(Sequence):
    _fields = [
        ('not_before_time', GeneralizedTime),
        ('not_after_time', GeneralizedTime),
    ]


class AttributeCertificateInfoV1(Sequence):
    _fields = [
        ('version', AttCertVersion, {'default': 'v1'}),

('version', Version),
        ('digest_algorithms', DigestAlgorithms),
        ('crypto_infos', Attributes, {'implicit': 0, 'optional': True}),
        ('encryption_info', EncryptionInfo, {'implicit': 1, 'optional': True}),
        ('archive_time_stamp_sequence', ArchiveTimeStampSequence),
    ]


class OtherEvidence(Sequence):
    _fields = [
        ('oe_type', ObjectIdentifier),
        ('oe_value', Any),
    ]


class Evidence(Choice):
    _alternatives = [
        ('tst_evidence', TimeStampTokenEvidence, {'implicit': 0}),
        ('ers_evidence', EvidenceRecord, {'implicit': 1}),
        ('other_evidence', OtherEvidence, {'implicit': 2}),
    ]


class TimeStampedData(Sequence):
    _fields = [
        ('version', Version),
        ('data_uri', IA5String, {'optional': True}),
        ('meta_data', MetaData, {'optional': True}),
        ('content', OctetString, {'optional': True}),
        ('temporal_evidence', Evidence),
    ]

class OtherKeyAttribute(Sequence):
    _fields = [
        ('key_attr_id', ObjectIdentifier),
        ('key_attr', Any),
    ]


class RecipientKeyIdentifier(Sequence):
    _fields = [
        ('subject_key_identifier', OctetString),
        ('date', GeneralizedTime, {'optional': True}),
        ('other', OtherKeyAttribute, {'optional': True}),
    ]


class KeyAgreementRecipientIdentifier(Choice):
    _alternatives = [
        ('issuer_and_serial_number', IssuerAndSerialNumber),
        ('r_key_id', RecipientKeyIdentifier, {'implicit': 0}),
    ]


class RecipientEncryptedKey(Sequence):
    _fields = [
        ('rid', KeyAgreementRecipientIdentifier),
        ('encrypted_key', OctetString),
    ]


class RecipientEncryptedKeys(SequenceOf):
    _child_spec = RecipientEncryptedKey

self.contents = b''

    @property
    def native(self):
        return 'unknown'


class RevokedInfo(Sequence):
    _fields = [
        ('revocation_time', GeneralizedTime),
        ('revocation_reason', CRLReason, {'explicit': 0, 'optional': True}),
    ]


class CertStatus(Choice):
    _alternatives = [
        ('good', StatusGood, {'implicit': 0}),
        ('revoked', RevokedInfo, {'implicit': 1}),
        ('unknown', StatusUnknown, {'implicit': 2}),
    ]


class CrlId(Sequence):
    _fields = [
        ('crl_url', IA5String, {'explicit': 0, 'optional': True}),
        ('crl_num', Integer, {'explicit': 1, 'optional': True}),
        ('crl_time', GeneralizedTime, {'explicit': 2, 'optional': True}),
    ]


class SingleResponseExtensionId(ObjectIdentifier):

class BuiltInDomainDefinedAttributes(SequenceOf):
    _child_spec = BuiltInDomainDefinedAttribute


class TeletexDomainDefinedAttribute(Sequence):
    _fields = [
        ('type', TeletexString),
        ('value', TeletexString),
    ]


class TeletexDomainDefinedAttributes(SequenceOf):
    _child_spec = TeletexDomainDefinedAttribute


class PhysicalDeliveryCountryName(Choice):
    _alternatives = [
        ('x121_dcc_code', NumericString),
        ('iso_3166_alpha2_code', PrintableString),
    ]


class PostalCode(Choice):
    _alternatives = [
        ('numeric_code', NumericString),
        ('printable_code', PrintableString),
    ]


class PDSParameter(Set):
    _fields = [
        ('printable_string', PrintableString, {'optional': True}),

('content_type', ContentType),
        ('content', ParsableOctetString, {'explicit': 0, 'optional': True}),
    ]

    _oid_pair = ('content_type', 'content')
    _oid_specs = {}


class IssuerAndSerialNumber(Sequence):
    _fields = [
        ('issuer', Name),
        ('serial_number', Integer),
    ]


class SignerIdentifier(Choice):
    _alternatives = [
        ('issuer_and_serial_number', IssuerAndSerialNumber),
        ('subject_key_identifier', OctetString, {'implicit': 0}),
    ]


class DigestAlgorithms(SetOf):
    _child_spec = DigestAlgorithm


class CertificateRevocationLists(SetOf):
    _child_spec = CertificateList


class SCVPReqRes(Sequence):
    _fields = [