fun power (n,0) = 1
  | power (n,k) = n * (power (n, k-1));

fun power2 (n,0) = 1
  | power2 (0,k) = 0 (* just makes it a bit faster *)
  | power2 (n,k) = if k < 0 then
          0
                   else
                       n * (power2 (n, k-1));

fun power3 (n,k) =
  if k = 0 then
      1
  else if n = 0 orelse k < 0 then
      0
  else
      n * (power3 (n, k-1));

fun power4 n 0 = 1
  | power4 0 k = 0
  | power4 n k = if k < 0 then
                     0
                 else
                     n * (power4 n (k-1));

(*fun powerAux (a,0,k) = a
  | powerAux (a,n,k) = powerAux(a*n,n,k-1);

fun power k n =
    if n = 0 orelse k < 0 then
        0
    else
        powerAux(1,n,k);*)