sub mmMxM {
    our( @A, @B );
    local( *A, *B ) = @_;
    my @C = map[0,0,0,0], 0..3;

    for my $i ( 0 .. 3 ) {
        for my $j ( 0 .. 3 ) {
            $C[$i][$j] += $A[$i][$_]*$B[$_][$j] for 0 .. 3;
        }
    }
    return \@C;
}