use Math::BigApprox qw/Fact/;
use Benchmark qw/cmpthese/;
use Memoize;

memoize( 'Fact', INSTALL => 'fastFact' );

our $test = 20000;

print "$test!\t=>\t", Fact($test), "\n";

cmpthese ( -5, {
    Fact        => sub{ my $a = Fact($test)     },
    fastFact    => sub{ my $a = fastFact($test) },
} );