sub mmMxN {
    our ( @M, @N );
    my $self = shift;
    local( *M, *N ) = @{ $self }{ M, N };
    die "Incompatible matrix dimensions" unless @{ $M } == @{ $N[0] };

    my @R = map[ (0) x @M ], 0 .. $#N[ 0 ];

    for my $i ( 0 .. $#M ) {
        for my $j ( 0 .. $#{ $N[0] } ) {
            $R[$i][$j] += $M[$i][$_] * $N[$_][$j] for 0 .. $#N;
        }
    }
    $self->{R} = \@R;
}