> function permgen (a, n)
>>       if n == 0 then
>>         printResult(a)
>>       else
>>         for i=1,n do
>>
>>           -- put i-th element as the last one
>>           a[n], a[i] = a[i], a[n]
>>
>>           -- generate all permutations of the other elements
>>           permgen(a, n - 1)
>>
>>           -- restore i-th element
>>           a[n], a[i] = a[i], a[n]
>>
>>         end
>>       end
>>     end
> function printResult (a)
>>       for i,v in ipairs(a) do
>>         io.write(v, " ")
>>       end
>>       io.write("\n")
>>     end
> permgen ({1,2,3,4}, 4)
2 3 4 1
3 2 4 1
3 4 2 1
4 3 2 1
2 4 3 1
4 2 3 1
4 3 1 2
3 4 1 2
3 1 4 2
1 3 4 2
4 1 3 2
1 4 3 2
2 4 1 3
4 2 1 3
4 1 2 3
1 4 2 3
2 1 4 3
1 2 4 3
2 3 1 4
3 2 1 4
3 1 2 4
1 3 2 4
2 1 3 4
1 2 3 4
>