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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 atlasdb_queue_zonefiles( con, db, start_block, zonefile_dir, recover=False, validate=True, end_block=None ):
 """ Queue all zonefile hashes in the BlockstackDB to the zonefile queue NOT THREAD SAFE Returns the list of zonefile infos queued, and whether or not they are present. """

    # populate zonefile queue
    total = 0
    if end_block is None:
        end_block = db.lastblock+1

    ret = []    # map zonefile hash to zfinfo

    for block_height in range(start_block, end_block, 1):
        
        # TODO: can we do this transactionally?
        zonefile_info = db.get_atlas_zonefile_info_at( block_height )
        for name_txid_zfhash in zonefile_info:
            name = str(name_txid_zfhash['name'])
            zfhash = str(name_txid_zfhash['value_hash'])
            txid = str(name_txid_zfhash['txid'])
            tried_storage = 0

            present = is_zonefile_cached( zfhash, zonefile_dir, validate=validate )
            zfinfo = atlasdb_get_zonefile( zfhash, con=con )
            if zfinfo is not None:
                tried_storage = zfinfo['tried_storage']

            if recover and present:
                log.debug('Recover: assume that {} is absent so we will reprocess it'.format(zfhash))
                present = False

            log.debug("Add %s %s %s at %s (present: %s, tried_storage: %s)" % (name, zfhash, txid, block_height, present, tried_storage) )
            atlasdb_add_zonefile_info( name, zfhash, txid, present, tried_storage, block_height, con=con )
            total += 1

            ret.append({
                'name': name,
                'zonefile_hash': zfhash,
                'txid': txid,
                'block_height': block_height,
                'present': present,
                'tried_storage': tried_storage
            })

    log.debug("Queued %s zonefiles from %s-%s" % (total, start_block, db.lastblock))
    return ret