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

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

    for my $i ( 0 .. $#M ) {
        for my $j ( 0 .. $#{ $N[0] } ) {
            $C[ $i ][ $j ] += $M[ $i ][ $_ ] * $N[ $_ ][ $j]  for 0 .. $#N;
        }
    }
    return \@C;
}

##</code><code>##

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};
}

##</code><code>##

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;
}