A Perl6 factorial postfix operator using recursive multi dispatch

postfix_factorial.p6

multi sub postfix:<!> (Int $i where $i < 0)  is tighter(&infix:<*>)  { fail "Not a Natural Number in Factorial" }
multi sub postfix:<!> (Int $i where 0|1)     is tighter(&infix:<*>)  { 1 }
multi sub postfix:<!> (Int $i where $i > 1)  is tighter(&infix:<*>)  { $i * ($i-1)! }

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";