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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 get_accounts(cls, soco=None):
 """Get all accounts known to the Sonos system. Args: soco (`SoCo`, optional):
 a `SoCo` instance to query. If `None`, a random instance is used. Defaults to `None`. Returns: dict: A dict containing account instances. Each key is the account's serial number, and each value is the related Account instance. Accounts which have been marked as deleted are excluded. Note: Any existing Account instance will have its attributes updated to those currently stored on the Sonos system. """


        root = XML.fromstring(cls._get_account_xml(soco))
        # _get_account_xml returns an ElementTree element like this:

        # <ZPSupportInfo type="User">
        #   <Accounts
        #   LastUpdateDevice="RINCON_000XXXXXXXX400"
        #   Version="8" NextSerialNum="5">
        #     <Account Type="2311" SerialNum="1">
        #         <UN>12345678</UN>
        #         <MD>1</MD>
        #         <NN></NN>
        #         <OADevID></OADevID>
        #         <Key></Key>
        #     </Account>
        #     <Account Type="41735" SerialNum="3" Deleted="1">
        #         <UN></UN>
        #         <MD>1</MD>
        #         <NN>Nickname</NN>
        #         <OADevID></OADevID>
        #         <Key></Key>
        #     </Account>
        # ...
        #   <Accounts />

        xml_accounts = root.findall('.//Account')
        result = {}
        for xml_account in xml_accounts:
            serial_number = xml_account.get('SerialNum')
            is_deleted = True if xml_account.get('Deleted') == '1' else False
            # cls._all_accounts is a weakvaluedict keyed by serial number.
            # We use it as a database to store details of the accounts we
            # know about. We need to update it with info obtained from the
            # XML just obtained, so (1) check to see if we already have an
            # entry in cls._all_accounts for the account we have found in
            # XML; (2) if so, delete it if the XML says it has been deleted;
            # and (3) if not, create an entry for it
            if cls._all_accounts.get(serial_number):
                # We have an existing entry in our database. Do we need to
                # delete it?
                if is_deleted:
                    # Yes, so delete it and move to the next XML account
                    del cls._all_accounts[serial_number]
                    continue
                else:
                    # No, so load up its details, ready to update them
                    account = cls._all_accounts.get(serial_number)
            else:
                # We have no existing entry for this account
                if is_deleted:
                    # but it is marked as deleted, so we don't need one
                    continue
                # If it is not marked as deleted, we need to create an entry
                account = Account()
                account.serial_number = serial_number
                cls._all_accounts[serial_number] = account

            # Now, update the entry in our database with the details from XML
            account.service_type = xml_account.get('Type')
            account.deleted = is_deleted
            account.username = xml_account.findtext('UN')
            # Not sure what 'MD' stands for.  Metadata? May Delete?
            account.metadata = xml_account.findtext('MD')
            account.nickname = xml_account.findtext('NN')
            account.oa_device_id = xml_account.findtext('OADevID')
            account.key = xml_account.findtext('Key')
            result[serial_number] = account
            # There is always a TuneIn account, but it is handled separately
            #  by Sonos, and does not appear in the xml account data. We
            # need to add it ourselves.
            tunein = Account()
            tunein.service_type = '65031'  # Is this always the case?
            tunein.deleted = False
            tunein.username = ''
            tunein.metadata = ''
            tunein.nickname = ''
            tunein.oa_device_id = ''
            tunein.key = ''
            tunein.serial_number = '0'
            result['0'] = tunein

        return result