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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 atlas_peer_get_zonefile_inventory( peer_hostport, peer_table=None ):
 """ What's the zonefile inventory vector for this peer? Return None if not defined """

    inv = None

    with AtlasPeerTableLocked(peer_table) as ptbl:
        if peer_hostport not in ptbl.keys():
            return None

        inv = ptbl[peer_hostport]['zonefile_inv']

    return inv