RosettaCodeData/Task/Power-set/Bracmat/power-set.bracmat

( ( powerset
  =   done todo first
    .   !arg:(?done.?todo)
      & (   !todo:%?first ?todo
          & (powerset$(!done !first.!todo),powerset$(!done.!todo))
        | !done
        )
  )
& out$(powerset$(.1 2 3 4))
);