public function rowSubtractScalar(int $mᵢ, int $k) : Matrix
{
if ($mᵢ >= $this->m) {
throw new Exception\MatrixException('Row to subtract does not exist');
}
$n = $this->n;
$R = $this->A;
for ($j = 0; $j < $n; $j++) {
$R[$mᵢ][$j] -= $k;
}
return MatrixFactory::create($R);
}