sub postfix:<!> (Int $n) {
      fail "Not a Natural Number in Factorial" if $n < 0;
      [*] 2..$n
  }

  use Test;
  isa-ok (-1)!, Failure, "Factorial for -1 fails";
  ok 0! == 1, "Factorial for 0";
  ok 1! == 1, "Factorial for 1";
  ok 5! == 120, "Factorial for a larger integer";