public function subtract($b)
    {
        switch (true) {
            case $b instanceof Matrix:
                $c = $this->subtractMatrix($b);
                break;

            case $b instanceof ColumnVector:
                $c = $this->subtractColumnVector($b);
                break;
                
            case $b instanceof Vector:
                $c = $this->subtractVector($b);
                break;

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

            default:
                throw new InvalidArgumentException('Cannot subtract a '
                    . gettype($b) . ' from a matrix.');
        }

        return $c;
    }