sub mmMxN {
    my $self = shift;
    die "Incompatible matrix dimensions" unless @{ $self->{M} } == @{ $self->{N}->[0] };

    $self->{R} = map[ (0) x @{ $self->{M} } ], 0 .. $#{ $self->{N}->[ 0 ] };

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