checkpoint <sectors>
    set checkpoint codes at sectors. 0 means no checkpoint. You will be
    asked for your country's password as authorization. Sectors which are
    checkpointed can be moved onto (navigated into for harbors) by other
    countries, if they know the checkpoint code. Note that highways and
    bridge spans can be moved onto UNLESS they are checkpointed. (If
    someone moves things onto your highway or bridge span and leaves them
    there, they become yours.) The presence of a checkpoint code is
    indicated by a '*' in the census listing.
Doc: society, sectors, ships
Help: census, move, navigate
