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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def texkeys(self, key, value):
 """Populate the ``texkeys`` key. Also populates the ``external_system_identifiers`` and ``_desy_bookkeeping`` keys through side effects. """

    def _is_oai(id_, schema):
        return id_.startswith('oai:')

    def _is_desy(id_, schema):
        return id_ and schema in ('DESY',)

    def _is_texkey(id_, schema):
        return id_ and schema in ('INSPIRETeX', 'SPIRESTeX')

    texkeys = self.get('texkeys', [])
    external_system_identifiers = self.get('external_system_identifiers', [])
    _desy_bookkeeping = self.get('_desy_bookkeeping', [])

    values = force_list(value)
    for value in values:
        ids = force_list(value.get('a', ''))
        other_ids = force_list(value.get('z', ''))
        schema = force_single_element(value.get('9', ''))

        for id_ in ids:
            id_ = id_.strip()
            if not id_:
                continue

            if _is_texkey(id_, schema):
                texkeys.insert(0, id_)
            elif _is_oai(id_, schema):
                continue  # XXX: ignored.
            elif _is_desy(id_, schema):
                _desy_bookkeeping.append({'identifier': id_})
            else:
                external_system_identifiers.insert(0, {
                    'schema': schema,
                    'value': id_,
                })

        for id_ in other_ids:
            id_ = id_.strip()
            if not id_:
                continue

            if _is_texkey(id_, schema):
                texkeys.append(id_)
            elif _is_oai(id_, schema):
                continue  # XXX: ignored.
            elif _is_desy(id_, schema):
                _desy_bookkeeping.append({'identifier': id_})
            else:
                external_system_identifiers.append({
                    'schema': schema,
                    'value': id_,
                })

    self['external_system_identifiers'] = external_system_identifiers
    self['_desy_bookkeeping'] = _desy_bookkeeping
    return texkeys