(* q2 *)
fun distribute a L = map(cons(a))(L);

(* q3 *)
fun eltcombos As L = reduce(append,map(fn a => distribute a L)(As),[]);

(* q4 *)
fun allcombos Ps = reduce(eltcombos,Ps,[[]]);

(* q5 *)
fun allcombo3 (ones,twos,threes) = map(tuple3)(allcombos([ones,twos,threes]));