(* ps *)

fun add_element e l = e::l;

fun ps1 [] = [[]]
   |ps1 (h::t) = let val p = ps1 t
                 in p @ map (add_element h) p end;

fun ps2 [] base = [base]
   |ps2 (e::s) base = (ps2 s base) @ 
                      (ps2 s (e::base));