public function notEqual($b)
    {
        switch (true) {
            case $b instanceof Vector:
                $c = $this->notEqualVector($b);
                break;

            case $b instanceof Matrix:
                $c = $this->notEqualMatrix($b);
                break;

            case is_int($b) or is_float($b):
                $c = $this->notEqualScalar($b);
                break;

            default:
                throw new InvalidArgumentException('Cannot compare vector'
                    . ' to a ' . gettype($b) . '.');
        }

        return $c;
    }