var int: factorial(int: n) =
  let {
    array[0..n] of var int: factorial;
    constraint forall(a in 0..n)(
      factorial[a] == if (a == 0) then
        1
      else
        a*factorial[a-1]
      endif
  )} in factorial[n];

var int: fac = factorial(6);
solve satisfy;
output [show(fac),"\n"];